Neuroscience · Unit 3: Sensory systems

Visual Cortex

How does the brain turn the retina's spots of contrast into edges, maps and objects? This lesson follows the optic nerve through the chiasm, the thalamus and the optic radiations to primary visual cortex, reads Hubel and Wiesel's recordings from cat and monkey cortex, and weighs the evidence for two cortical streams. Afterwards you can predict the field defect from a lesion at each point on the pathway, read an orientation tuning curve, and say what a column map is and in which species it exists.

Watch first Why a bar beats a spot · 2:49

Cut one optic nerve and one eye goes blind. Cut one optic tract, a few centimetres further back, and both eyes lose the same half of the world.

The difference comes from one crossing. The retina sends the brain a mosaic of centre-surround signals. Primary visual cortex, called V1 or striate cortex, turns those spots into oriented edges, lays them out as a map, and hands them on to dozens of further areas. Here you follow the pathway, read V1 recordings, compute a tuning width and a cortical distance, and weigh the evidence for two streams. Phototransduction belongs to senses and cGMP; receptive-field logic to sensory coding.

From eye to cortex: one crossing, four defects

The visual field is the part of the world one eye sees while it holds still. It has a nasal half (toward the nose) and a temporal half (toward the temple). The lens inverts the image, so the temporal field falls on the nasal retina and the upper field on the lower retina. Most mistakes here come from mixing field and retina.

Ganglion-cell axons gather at the optic disc and leave the eye as the optic nerve. At the optic chiasm only the fibres from the nasal half of each retina cross to the other side. Temporal-retina fibres stay ipsilateral (on the same side). Behind the chiasm, each optic tract therefore carries the contralateral (opposite) visual hemifield from both eyes: the left tract carries everything to the right of the point of gaze. The left hemisphere does not serve the right eye; it serves the right half of the world, seen by both eyes.

The main target of the tract is the lateral geniculate nucleus (LGN) of the thalamus; other branches reach the superior colliculus, pretectum and hypothalamus. A nucleus is a cluster of neuron cell bodies in the central nervous system; the LGN keeps the two eyes’ inputs in separate layers. LGN axons fan out as the optic radiations to V1, along the calcarine sulcus at the back of the occipital lobe. The radiations split. Fibres for the upper field sweep forward through the temporal lobe in Meyer’s loop and end below the calcarine sulcus; fibres for the lower field run through the parietal lobe and end above it.

Lesion logic from the nervous system map gives four defects, described for human anatomy (Patel and colleagues, 2021). Each “can” occur; a partial lesion gives a partial defect.

  • Optic nerve: loss of the whole field of the eye on the same side.
  • Chiasm, centre: the crossing nasal fibres of both eyes are cut, so both temporal fields are lost, a bitemporal hemianopia.
  • Optic tract, radiation or V1 on one side: the opposite half of the field in both eyes, a homonymous hemianopia (homonymous means the same side of the field in each eye).
  • One branch of the radiation: a quarter field, a quadrantanopia. Meyer’s loop damage gives an upper quarter defect on the opposite side, reported after temporal-lobe surgery (Powell and colleagues, 2005); parietal radiation damage gives a lower quarter.

Simple and complex receptive fields

A receptive field is the region of the visual field where a stimulus changes a neuron’s firing. David Hubel and Torsten Wiesel recorded extracellularly from single cortical cells in anaesthetized, light-adapted cats while they projected spots, slits and bars onto a screen. Their 1962 paper in the Journal of Physiology described two main classes.

Simple cells have separate ON subregions (where light excites) and OFF subregions (where light inhibits and its removal excites), lying side by side. Light that covered an ON and an OFF region together tended to cancel. Because the subregions are elongated, the orientation of a bar was critical: a bar aligned with the ON strip drove the cell, a bar across it barely did.

Complex cells kept the orientation preference but tolerated position. A correctly oriented bar drove them over much of the field, and no separate ON and OFF regions could be mapped. They do not ignore orientation.

Hubel and Wiesel suggested how these fields might be built: several LGN cells with centres in a row converge on a simple cell, and simple cells of one orientation but different positions converge on a complex cell. They offered it as a suggestion. The recordings measured response properties; the wiring was a proposal, and how far every complex cell follows it has been debated since.

Worked problem: reading a tuning curve. This is a made-up V1 recording. A bar at the cell’s best position is shown at six orientations, many trials each. Mean rates: 0 degrees, 2 spikes/s; 30 degrees, 6; 60 degrees, 24; 90 degrees, 40; 120 degrees, 22; 150 degrees, 5. The spontaneous rate, with no bar, is 2 spikes/s. Assume the response varies linearly between sampled orientations.

Step 1: preferred orientation. 90 degrees (40 spikes/s).

Step 2: half height. Measure from the spontaneous rate: 2 + (40 − 2)/2 = 2 + 19 = 21 spikes/s.

Step 3: where the curve crosses 21. On the rising side, between 30 degrees (6) and 60 degrees (24), 21 is 3 below the 60 degree value, out of a rate difference of 18 across the interval: 60 − 30 × 3/18 = 60 − 5 = 55 degrees. On the falling side, between 120 degrees (22) and 150 degrees (5), 21 is 1 below the 120 degree value, out of a rate difference of 17: 120 + 30 × 1/17 = 121.8 degrees.

Step 4: width. 121.8 − 55 = 66.8, about 67 degrees full width at half height.

Step 5: orientation index. Take 0 degrees, 90 degrees from the preferred, as orthogonal:

orientation index = (R_pref − R_orth) / (R_pref + R_orth) = (40 − 2)/(40 + 2) = 38/42 = 0.90

An index near 1 means the cell strongly prefers its best orientation over the orthogonal one; it does not by itself give the tuning width. With samples every 30 degrees, the width is only an interpolated estimate.

Maps, magnification and columns, and where columns are absent

V1 is retinotopic: neighbouring points in the visual field excite neighbouring points in cortex, so the field is laid out as a map. In humans the fovea, the centre of gaze, is represented at the occipital pole, posterior to the periphery; the upper field lies below the calcarine sulcus and the lower field above it. How that map is wired up during development is covered in topographic maps.

The map is distorted. Cortical magnification is the length of cortex, in millimetres, given to one degree of visual field. It is largest at the fovea and falls steeply with eccentricity. Tootell and colleagues (1988) mapped macaque V1 with 2-deoxyglucose while the animals viewed patterns of known position, and reported a maximum near the foveal representation of about 15 mm per degree.

Worked problem: a cortical distance. Assume a locally constant magnification of 15 mm per degree near the macaque fovea. Two points 0.20 degrees apart lie

distance on cortex = M × separation = 15 mm/degree × 0.20 degrees = 3.0 mm

apart. Check: 3.0 mm ÷ 15 mm/degree = 0.20 degrees. This holds only near the fovea; never apply it across the field.

On top of the map sit columns: groups of cells, running through the thickness of the cortex, that share a property. In cat, Hubel and Wiesel (1962) found that cells along a penetration at right angles to the surface tended to share an orientation preference, and that most of the cells they sampled were influenced by both eyes. In lightly anaesthetized macaque and spider monkeys (1968) they found orientation columns and ocular-dominance columns, groups of cells preferring one eye, with boundaries that seemed to be independent of each other. The 1962 results are cat; the 1968 columns are monkey. How eye closure in young animals reshapes ocular dominance belongs to ocular dominance and critical periods, and prenatal activity to spontaneous activity.

Columns are not universal. Ohki, Reid and colleagues (2005) imaged single cells with two-photon calcium microscopy. In cat area 18, preferences were so orderly that direction-column borders were one or two cells wide. In rat V1, cells were orientation tuned but had no discernible local map: neighbours preferred unrelated orientations, a layout called salt-and-pepper. Tuning does not need a map. In human fMRI maps, a population receptive field describes pooled activity of many neurons (Benson and colleagues, 2018, 181 participants), not one cell’s field.

Two streams: what the evidence shows

In 1982 Leslie Ungerleider and Mortimer Mishkin summarized monkey evidence into two systems. Inferior temporal lesions impaired object discrimination; posterior parietal lesions impaired a landmark task that depends on spatial position. They proposed a ventral stream, occipitotemporal, for object vision, and a dorsal stream, occipitoparietal, for spatial vision: “what” and “where”. It was a working hypothesis.

Patient D.F. changed the question. She has visual form agnosia and reports the width and orientation of objects poorly. Goodale, Milner, Jakobson and Carey (1991) gave her plaques of equal area but different widths. Her width matching was poor, yet when she reached to pick the same plaques up, her grip opened in proportion to width. The object was the same; only the use of vision differed. Goodale and Milner proposed in 1992 that the ventral stream serves perception and the dorsal stream the visual control of action.

Keep its limits in view. D.F. is one patient, and one dissociation does not localize a circuit. The streams are densely interconnected, and no area has a single function. Even V1 is not a pure camera: in mouse, part of its ongoing activity is related to the animal’s movements (Stringer and colleagues, 2019), and feedback from the higher area LM depends on which part of the visual field each region represents (Fişek and colleagues, 2023). Attention returns in attention and decisions, and recording thousands of cells in large-scale neuroscience.

Design problem

Four patients each have one lesion of the visual pathway. (i) The whole field of the left eye is lost. (ii) The temporal half of the field is lost in both eyes. (iii) The right half of the field is lost in both eyes. (iv) The upper right quarter of the field is lost in both eyes. For each, locate the lesion and name the retinal halves whose output is lost.

Visual pathway lesion map: where is the lesion?⚙ original · interactive
Lesion site
Side (the patient’s)

Field defect
Retinal parts whose signal is lost

fibres from the nasal half of a retina fibres from the temporal half dashed grey: output lost pink V1 half: none of its input arrives (in this sketch)

Lower panel: each eye’s field as the patient sees it, the patient’s left on the viewer’s left. Hatched = lost.

In this sketch: a schematic seen from above, not an anatomical section; complete lesions only; human anatomy as the lesson gives it; each lost field part is drawn whole; the stacked boxes are a schematic of above and below the calcarine sulcus, not to scale.

Rule to keep: only nasal-retina fibres cross, so a lesion before the chiasm costs one eye its field, at the chiasm centre both temporal fields, and behind it the opposite half or quarter of the field in both eyes. Partial lesions give partial defects, close matching between the eyes and sparing of central vision are tendencies only, and this is anatomical reasoning, not clinical advice.

One way to do it

Draw both retinal images first (inverted both ways), then the fibres.

(i) The left optic nerve, before the chiasm: all of the left retina is cut off.

(ii) Both temporal fields: the temporal field falls on the nasal retina, and the nasal fibres of both eyes meet at the chiasm. A lesion at its centre cuts the crossing nasal fibres of both eyes.

(iii) The right half in both eyes: the right field falls on the left half of each retina, the temporal half of the left eye and the nasal half of the right eye. Those fibres join in the left optic tract, so the lesion is in the left tract, left LGN, left optic radiation or left V1.

(iv) The upper right quarter in both eyes: the upper right field falls on the lower left quarter of each retina (lower temporal in the left eye, lower nasal in the right eye). Upper-field fibres run in left Meyer’s loop to the cortex below the left calcarine sulcus, so either site fits.

Partial lesions give partial defects, and close matching between the eyes or sparing of central vision are tendencies only. This is anatomical reasoning, not clinical advice.

How we measure it

Extracellular single-unit recording

A fine electrode placed near a neuron records its action potentials without entering the cell. Moving and rotating a stimulus while counting spikes maps the cell's receptive field and tuning. It samples one cell at a time and favours large, active cells.

2-deoxyglucose metabolic mapping

A radioactive glucose analogue is taken up by active neurons and trapped. If an animal views a chosen pattern during the infusion, autoradiographs of the cortex show where activity was high, giving a map of the whole area for that stimulus. It measures metabolism summed over the session, not spikes or connections.

Two-photon calcium imaging

A calcium-sensitive indicator in cortical neurons is imaged through a cranial window with infrared laser light that excites only the focal plane. Hundreds to thousands of cells can be followed at once, so the preferences of neighbouring cells can be compared directly.

Lesions and matched perception and action tasks

In monkeys, a cortical area is removed and performance on tasks that depend on object identity or spatial position is compared. In patients, the same object is used for a perceptual report and for an action, so a dissociation points to different uses of the same visual input.

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Hearing and Balance

The same kind of cell tells you that a violin is playing an A and that your head is turning left. How does one cell type do both jobs?

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David Hubel

David Hubel 1926–2013

With Torsten Wiesel, recorded single cells in the visual cortex of anaesthetized cats and later monkeys, described simple and complex receptive fields and their orientation selectivity, and mapped orientation and ocular-dominance columns.

Hubel was born in Windsor, Ontario, and took his medical degree at McGill University in 1951. After clinical training and a period of research on cortical recording, he joined Stephen Kuffler's group at Johns Hopkins, where his collaboration with Torsten Wiesel began. Kuffler had shown that retinal ganglion cells have centre-surround receptive fields. The obvious next question was what the cortex does with them. Hubel and Wiesel recorded extracellularly from single cells in the visual cortex of anaesthetized, light-adapted cats while they projected spots and bars of light onto a screen in front of the animal. Spots, which drive the retina well, often did little. Bars at the right orientation drove cells hard. Their 1959 and 1962 papers in the Journal of Physiology sorted cortical cells into simple and complex types and suggested how one might be built from the other. In 1968 they extended the work to macaque and spider monkeys. Hubel and Wiesel shared the 1981 Nobel Prize in Physiology or Medicine with Roger Sperry. Their method, a bar of light and an electrode, made receptive-field mapping the standard way to ask what a cortical cell encodes.

✓ verbatim Hubel & Wiesel, J. Physiol. 160:106–154 (1962); Hubel & Wiesel, J. Physiol. 195:215–243 (1968); Hubel, Nobel Lecture, 8 December 1981 (nobelprize.org).
Torsten Wiesel

Torsten Wiesel born 1924

With David Hubel, showed that cells in cat and monkey visual cortex respond best to oriented edges, that most cells in the cat sample were driven by both eyes, and that cells with similar preferences are grouped into columns.

Wiesel was born in Uppsala, Sweden, and took his medical degree at the Karolinska Institute in 1954. He moved to Johns Hopkins to work in Stephen Kuffler's group, where he and David Hubel began recording together. In the 1962 paper on cat visual cortex they did more than classify cells. They tested each cell through each eye separately and reported that most of the cells in their sample were influenced by both eyes, often unequally. Their electrode tracks showed that cells met along a penetration made at right angles to the cortical surface tended to share an orientation preference, which they described as columns. The 1968 study in monkeys mapped orientation and eye preference along the same penetrations and found that the two kinds of column had boundaries that seemed to be independent. Wiesel and Hubel also used eye closure in kittens to study how visual experience shapes these inputs, work covered in the developmental neuroscience course. Their binocular measurements gave later work on the development of vision a quantity to track.

✓ verbatim Hubel & Wiesel, J. Physiol. 160:106–154 (1962); Wiesel, Nobel Lecture, 8 December 1981 (nobelprize.org); Lindau Nobel Laureate Meetings CV.
Leslie Ungerleider

Leslie Ungerleider 1946–2020

With Mortimer Mishkin, brought together monkey lesion and anatomical evidence into the proposal of two cortical visual systems: a ventral occipitotemporal pathway for object vision and a dorsal occipitoparietal pathway for spatial vision.

Ungerleider worked in Mortimer Mishkin's laboratory at the National Institute of Mental Health, where monkeys were trained on two kinds of visual task. In an object discrimination, the monkey learned which of two objects covered a food well. In a landmark task, it learned to choose the food well closer to a landmark, whatever the objects looked like. Removing inferior temporal cortex impaired the first kind of task; removing posterior parietal cortex impaired the second. Tracing the connections from striate cortex outward gave two routes, one running down into the temporal lobe and one running up into the parietal lobe. In a 1982 book chapter, "Two cortical visual systems", Ungerleider and Mishkin proposed that the ventral route serves object vision and the dorsal route serves spatial vision, a scheme soon shortened to "what" and "where". It was a synthesis and a working hypothesis. Ungerleider later turned to functional imaging in humans with colleagues. The framework gave the dozens of extrastriate areas a first organizing principle that experiments could test.

✓ verbatim Ungerleider & Mishkin, 'Two cortical visual systems', in Ingle, Goodale & Mansfield (eds), Analysis of Visual Behavior, MIT Press (1982), pp. 549–586.
Melvyn Goodale

Melvyn Goodale

With David Milner, Lorna Jakobson and David Carey, showed that patient D.F. could scale her grasp to objects whose width she could not report, and with Milner proposed that the two streams differ in what vision is used for: perception versus action.

D.F. is a patient with visual form agnosia after brain damage: her colour vision was largely preserved, but she had great difficulty reporting the shape, size or orientation of objects. Goodale and his colleagues noticed that her actions seemed better than her reports. To test it, they gave her flat plaques of equal area that differed in width. Asked to show the width of a plaque by opening her finger and thumb, she did poorly. Asked to reach out and pick the same plaque up, she opened her hand in proportion to its width as she reached, as control participants did. The task, the objects and the viewing were the same; only the use of the information differed. Their 1991 letter in Nature interpreted the result as separate processing for different uses of visual information, not for different subsets of it. In 1992 Goodale and Milner proposed that the ventral stream serves perception and the dorsal stream serves the visual control of action, a revision of "what versus where". The work made the response task itself a variable in visual neuroscience.

✓ verbatim Goodale, Milner, Jakobson & Carey, Nature 349:154–156 (1991); Goodale & Milner, Trends Neurosci. 15:20–25 (1992).
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At the optic chiasm, which retinal axons cross to the other side of the brain?
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Every structure, slide, simulation, and clip on this page is listed below with the source it came from. None of the 3D structures is drawn by hand or by AI: each one loads by accession ID from RCSB PDB or PubChem. This panel is generated from the lesson's own data file, so what it lists is what the page loads.

Simulations

  • Where is the lesion? · Original interactive — Biochemistrypedia (Deppmann Lab).

Scientist profiles & quotes

  • David Hubel — Hubel & Wiesel, J. Physiol. 160:106–154 (1962); Hubel & Wiesel, J. Physiol. 195:215–243 (1968); Hubel, Nobel Lecture, 8 December 1981 (nobelprize.org).
  • Torsten Wiesel — Hubel & Wiesel, J. Physiol. 160:106–154 (1962); Wiesel, Nobel Lecture, 8 December 1981 (nobelprize.org); Lindau Nobel Laureate Meetings CV.
  • Leslie Ungerleider — Ungerleider & Mishkin, 'Two cortical visual systems', in Ingle, Goodale & Mansfield (eds), Analysis of Visual Behavior, MIT Press (1982), pp. 549–586.
  • Melvyn Goodale — Goodale, Milner, Jakobson & Carey, Nature 349:154–156 (1991); Goodale & Milner, Trends Neurosci. 15:20–25 (1992).

Quotes are verbatim from the cited source. Profile text is original to this site and was checked against the sources cited.

Animated explainer

Why a bar beats a spot · narration: an AI narrator voice (Gemini, Charon) reading narration written from this lesson's text · animation: AI-generated in code (Manim) by Claude, a schematic with no drawn molecular structures, plots computed from their equations · captions from the narration script.

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Like I'm five

Your eyes send pictures to the back of your brain. Each side of your brain looks after the opposite side of the world, using both eyes. Some brain cells there like lines tilted one way and ignore lines tilted another way. After that, the picture goes two ways: one helps you know what a thing is, and the other helps you reach for it.
High school

The optic nerves meet at the optic chiasm, where fibres from the inner (nasal) half of each retina cross over. From there, each side of the brain receives the opposite half of the visual world from both eyes. The signal passes through a relay in the thalamus to the primary visual cortex at the back of the head. Many cells there respond best to an edge at one particular angle, and nearby cells tend to share preferences. The centre of gaze takes up far more cortex than the edges of vision. Beyond this first area, one stream of areas in the temporal lobe is important for recognizing objects and another in the parietal lobe for space and action, though the two constantly work together.
College

Retinal ganglion-cell axons form the optic nerve; at the chiasm nasal-retina fibres cross, so each optic tract carries the contralateral hemifield to the lateral geniculate nucleus, which projects through the optic radiations (upper field through Meyer's loop in the temporal lobe, lower field through the parietal lobe) to V1. Lesion sites therefore predict monocular loss, bitemporal hemianopia, homonymous hemianopia or quadrantanopia. V1 is retinotopic with strong foveal magnification. In anaesthetized cat, Hubel and Wiesel found simple cells with separate ON and OFF subregions and complex cells that keep orientation tuning across positions, and proposed that convergence builds each from the level below. Monkey V1 has orientation and ocular-dominance columns; rat V1 is tuned with a salt-and-pepper layout. Ventral and dorsal streams were proposed for object and spatial vision and later reframed as perception and action.
Graduate

Orientation selectivity in V1 is classically proposed to emerge from inputs with little orientation preference, and Hubel and Wiesel's 1962 scheme (aligned geniculate centres converging on a simple cell, simple cells of one orientation converging on a complex cell) remains a proposal whose details are still argued: the roles of intracortical excitation and inhibition, and whether all complex cells sit above simple cells, are not settled. Columnar architecture is species dependent. Macaque and cat show orderly orientation maps, while rat V1 neurons are tuned without local map structure (Ohki et al., 2005), so columns are not required for tuning. Magnification near the macaque fovea reached about 15 mm per degree in 2-deoxyglucose maps (Tootell et al., 1988), and human pRF maps from fMRI describe pooled activity, not single fields. In mouse, part of V1's ongoing activity is related to the animal's movements (Stringer et al., 2019), and feedback from area LM is spatially structured (Fişek et al., 2023). Open question: what computation, if any, depends on having an orientation map rather than salt-and-pepper tuning?
1981, not 1982

Hubel and Wiesel shared the 1981 Nobel Prize in Physiology or Medicine with Roger Sperry, who received his share for work on the functional specialization of the cerebral hemispheres in split-brain patients. The year is sometimes given as 1982, including in at least one widely used online textbook chapter. The Nobel archive is the source to trust. The pairing of the two awards is a reminder that the same prize honoured single-cell recordings in anaesthetized animals and behavioural tests in a small number of human patients, two very different kinds of evidence about the cortex.

— Nobel Prize in Physiology or Medicine 1981, prize summary, nobelprize.org.
Tuned, but not in columns

For decades, orientation columns were taught as the way V1 is built. Ohki, Reid and colleagues loaded a calcium indicator into cortical neurons and imaged hundreds of cells at once with two-photon microscopy in living animals. In cat area 18, preferences were ordered so precisely that the border between direction columns was one or two cells wide. In rat V1, cells were well tuned to orientation, yet neighbours preferred unrelated orientations. The same method gave both answers, so the difference lay in the animals, not the technique. Tuning and a map of tuning had come apart.

— Ohki, Chung, Ch'ng, Kara & Reid, Nature 433:597–603 (2005).
A grasp that knew more than she could say

Goodale and colleagues filmed D.F.'s hand as she reached for flat plaques of equal area but different widths. When asked to indicate a plaque's width with her finger and thumb, her estimates bore little relation to the real width. When she reached out to pick it up, the opening of her hand before contact grew with the plaque's width, as it did in control participants. One patient cannot map a whole cortical system, and the authors framed the interpretation conditionally. But the experiment held the object fixed and changed only the task, which is what made it persuasive.

— Goodale, Milner, Jakobson & Carey, Nature 349:154–156 (1991).

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