Occipital Lobe

Abstract

The occipital lobe is the most posterior lobe of the cerebral cortex, and it is the brain's principal visual processing region. Almost all of its surface is devoted to sight, from the primary visual cortex buried in the calcarine sulcus to a mosaic of extrastriate maps that parse orientation, colour, motion, form and scene. Its defining property is retinotopy: the retinal image is mapped onto cortex point for point, with the central few degrees magnified far out of proportion to their visual angle. Single-unit recording revealed neurons tuned to edge orientation, arranged in orderly columns; functional imaging later resolved a dozen distinct field maps feeding the dorsal and ventral streams. Focal damage produces retinotopically predictable blind regions, and the residual capacities of the blind field expose how much vision the lobe supports.

Keywords: occipital lobe, primary visual cortex, retinotopy, cortical magnification, blindsight

The occipital lobe occupies the back of each cerebral hemisphere, behind the parietal and temporal lobes and separated from the cerebellum by the tentorium. More than any other lobe it is functionally singular: sight dominates it so completely that a lesion anywhere within it removes a defined patch of the visual world. This article traces the lobe from its gross anatomy through the retinotopic and columnar organisation of the primary visual cortex, the specialisation of the surrounding extrastriate areas, and the clinical syndromes — hemianopia, quadrantanopia and blindsight — that follow when parts of it are destroyed.

## What the Occipital Lobe Is

The occipital lobe is the smallest of the four cortical lobes and the one whose function is most nearly uniform. It sits at the posterior pole of the hemisphere, bounded in front by the parieto-occipital sulcus on the medial surface and by an arbitrary line to the preoccipital notch laterally. Its most important landmark is the calcarine sulcus, a deep horizontal fissure on the medial wall along which the primary visual cortex is folded. Because the striate cortex dips into this sulcus, most of the primary map is hidden from a lateral view of the brain.

Visual information reaches the lobe by a long, ordered relay. The retina projects through the optic nerve and chiasm to the lateral geniculate nucleus of the thalamus, which in turn sends the optic radiation sweeping back to terminate in V1. Gordon Holmes, working from the localised occipital wounds of soldiers in the First World War, was the first to chart how positions in the visual field correspond to positions on the human occipital cortex, and his maps remain broadly accurate a century later (#ref-holmes-1918). The crossing at the chiasm means each occipital lobe represents the contralateral visual field, and the fold of the calcarine sulcus means the upper field is represented below it and the lower field above.

Figure 1

The occipital lobe on the medial surface, with the primary visual cortex along the calcarine sulcus.

## Types of Occipital Lobe

In the Medical Subject Headings (MeSH) vocabulary the occipital lobe is filed under the cerebral cortex, and it has one narrower descriptor: the visual cortex. MeSH is an indexing classification, not a functional parcellation — its single child stands for the whole visual territory of the lobe rather than enumerating each retinotopic map that neuroscientists distinguish. The subdivisions that matter for cognition (V1 through V5, the ventral and dorsal areas) are orthogonal to this indexing hierarchy; they are introduced in the sections that follow rather than in the table below.

## Retinotopic Organization

The single most important organising principle of the occipital lobe is retinotopy: neighbouring locations in the visual field are represented at neighbouring locations on the cortical sheet, so the cortex carries a distorted map of the retina. The distortion is systematic. The central few degrees of the visual field — the part imaged on the fovea — command a wholly disproportionate share of cortical surface, an effect called cortical magnification. Roughly half of V1 is devoted to the central ten degrees, even though those degrees are a small fraction of the visual field's extent.

Functional magnetic resonance imaging turned this principle into a working tool. Sereno and colleagues used phase-encoded retinotopic mapping to reveal the borders of multiple visual areas in the living human brain, each a mirror-reversed hemifield representation abutting the next (#ref-sereno-1995). Wandell and colleagues catalogued more than a dozen such field maps and showed how their eccentricity and polar-angle gradients tile the occipital and neighbouring cortex (#ref-wandell-2007), and later work made the imaging of these maps routine (#ref-wandell-winawer-2011). Probabilistic atlases now assign each cortical location a likelihood of belonging to a given map across individuals (#ref-wang-2015), and template models predict a person's retinotopy from anatomy alone with striking accuracy (#ref-benson-winawer-2018).

## Receptive Fields and Feature Selectivity

A retinotopic address tells only where a neuron looks, not what it responds to. The answer to the second question came from single-unit recording in the cat and monkey striate cortex by David Hubel and Torsten Wiesel. They found that most V1 neurons do not respond to diffuse light at all but to an edge or bar of a particular orientation, drifting through a small region of the visual field — the cell's receptive field (#ref-hubel-wiesel-1968). Simple cells respond to an edge in a fixed position and phase; complex cells respond to the same orientation anywhere within their field.

Hubel and Wiesel also uncovered the anatomical order behind this selectivity. Preferred orientation shifts smoothly as an electrode passes tangentially through the cortex, cycling through the full range every millimetre or so in an orientation column; interleaved with these are ocular-dominance columns, alternating stripes that favour one eye or the other. A small patch of cortex that contains a full cycle of orientations for both eyes, covering one point in the visual field, is a functional module sometimes called a hypercolumn. This columnar architecture, discovered by recording one cell at a time, is the microscopic counterpart of the macroscopic retinotopic map.

## Functional Specialization and the Visual Streams

Beyond V1 the cortex fragments into functionally distinct extrastriate areas. Semir Zeki, recording in the rhesus monkey, showed that these areas are not redundant copies of V1 but are specialised: one region responded selectively to the wavelength composition of a stimulus, another to the direction of motion, establishing that different attributes of a scene are analysed in anatomically separate compartments (#ref-zeki-1978). In the primate these map onto area V4, biased toward colour and form, and area V5/MT, biased toward motion.

The specialised areas are grouped into two great processing streams. Goodale and Milner argued that the ventral stream, running from the occipital lobe into the temporal lobe, computes the identity of objects for perception, while the dorsal stream, running toward the parietal lobe, computes their spatial layout for the guidance of action — a division between vision-for-perception and vision-for-action (#ref-goodale-milner-1992). Within the ventral stream, high-level regions become selective for whole categories: Kanwisher and colleagues identified a patch of the fusiform gyrus that responds far more to faces than to other objects, the fusiform face area (#ref-kanwisher-1997). The functional architecture of this ventral territory, its category-selective regions and their role in recognition, has been mapped in detail (#ref-grill-spector-weiner-2014), and the neural basis of face perception in particular is now finely resolved (#ref-grill-spector-2017). Scene-selective regions carry both low-level and high-level scene properties (#ref-groen-2017), and the perception of places and spatial context is a distinct ventral function in its own right (#ref-epstein-baker-2019).

## Visual Field Defects and Blindsight

Because the occipital lobe maps the visual field so faithfully, destroying part of it removes a corresponding part of vision, and the shape of the loss betrays the site of the lesion. A lesion of one entire striate cortex produces a homonymous hemianopia: blindness in the contralateral half of the field of both eyes. Because the calcarine sulcus splits the map top from bottom, damage confined to one bank produces a quadrantanopia — loss of the upper contralateral quadrant for a lesion below the sulcus, the lower quadrant for a lesion above it. A still smaller lesion carves out a scotoma, an island of blindness within an otherwise intact field, at the retinotopic location the damaged patch represents. Central vision is often spared, a clinical sign called macular sparing, in part because the large foveal representation at the occipital pole enjoys a dual blood supply.

The residual vision of the blind field is one of the most instructive findings in the field. Weiskrantz and colleagues showed that a patient with a restricted occipital lesion could locate and discriminate stimuli presented in the blind hemifield well above chance, despite reporting no conscious experience of them — the phenomenon of blindsight (#ref-weiskrantz-1974). It implies that visual information can reach action systems by routes that bypass V1, and it bears directly on what the striate cortex contributes to conscious sight. Ffytche and Zeki revisited this question and argued that under some conditions the primary visual cortex, and feedback to it, are not strictly necessary for a conscious visual percept (#ref-ffytche-zeki-2011). Holmes's wartime maps of occipital gunshot wounds were the first systematic demonstration of how lesion location predicts the field defect, and they founded this whole clinical tradition (#ref-holmes-1918).

## Worked Example

Cortical magnification can be made quantitative. A widely used description of human V1 is the inverse-linear form for the linear magnification factor, the millimetres of cortex per degree of visual angle at eccentricity E:

M(E) = A / (E + E₀)

with representative constants A = 17.3 mm·deg and E₀ = 0.75 deg. The cortical distance from the foveal representation out to eccentricity E is the integral of M:

d(E) = A · ln((E + E₀) / E₀)

Evaluate this at one and at ten degrees. At E = 1°, d = 17.3 · ln(1.75 / 0.75) = 17.3 · ln(2.333) = 17.3 · 0.8473 = 14.66 mm. At E = 10°, d = 17.3 · ln(10.75 / 0.75) = 17.3 · ln(14.333) = 17.3 · 2.6626 = 46.06 mm. The central one degree of the field therefore occupies about 14.66 mm of cortex, while the entire ring from one to ten degrees occupies only 46.06 − 14.66 = 31.40 mm. The innermost degree — a tenth of the way out to ten degrees — claims 14.66 / 46.06 ≈ 32 per cent of the cortical distance. That is cortical magnification in a single number: acuity is high at the fovea because so much cortex is spent on it. The first demonstration above computes d(E) from the same formula, so its readout agrees with the arithmetic here.

## Discussion

The occipital lobe is unusual among cortical lobes in how directly structure predicts function. Retinotopy makes the visual field literally legible on the cortical sheet, so a lesion's location can be inferred from a perimetry chart and a field defect can be predicted from a scan. That transparency is why the earliest human functional localisation came from occipital wounds, and why the occipital cortex became the proving ground for functional imaging: a stimulus whose retinal position is known drives a cortical location that can be found and measured.

Yet the lobe is not merely a screen. The progression from oriented edges in V1 to category-selective responses in the ventral stream shows a hierarchy of increasing abstraction, and the split into dorsal and ventral streams shows that a single retinal image is analysed for different purposes in parallel. Blindsight complicates the picture further by revealing that some visual function survives the loss of V1 entirely, which forces a distinction between the information the lobe carries and the conscious experience that normally accompanies it. The occipital lobe is thus both the most orderly and, at its edges, one of the most theoretically contested regions of the cortex.

## Current Directions

The retinotopic map is now a computational object rather than a hand-drawn chart. Template models that predict an individual's field maps from cortical anatomy alone have made it possible to define visual areas without a dedicated mapping experiment, and Bayesian methods combine such templates with limited measurements to reconstruct a person's full retinotopy (#ref-benson-winawer-2018). Probabilistic atlases give each cortical vertex a graded membership in each visual area, replacing sharp borders with population statistics (#ref-wang-2015). On the specialisation side, the questions have shifted from where a category is represented to how scene and object information is structured: recent work dissects the contributions of low-level image statistics and high-level semantic properties to scene-selective responses (#ref-groen-2017), and reviews of place and scene perception treat the ventral occipito-temporal cortex as a system with its own computational goals rather than a bank of detectors (#ref-epstein-baker-2019). Together these directions are turning a descriptive map into a predictive, individualised model of visual cortex.

## Common Misconceptions

The occipital lobe simply receives an image, like a screen.

Even V1 does not represent a picture; its neurons respond to oriented edges within small receptive fields, and downstream areas extract motion, colour and object identity. Vision is analysis, not display (#ref-hubel-wiesel-1968).

Each occipital lobe handles one eye.

The division is by visual field, not by eye. Because of the optic chiasm, each occipital lobe represents the contralateral half of the field seen by both eyes; the eyes are separated at a finer grain, in ocular-dominance columns within V1 (#ref-hubel-wiesel-1968).

Losing the primary visual cortex abolishes all vision in the affected field.

Some patients retain above-chance detection and localisation in a cortically blind field without conscious sight, a residual capacity called blindsight that depends on pathways bypassing V1 (#ref-weiskrantz-1974).

## Glossary

Blindsight.

Above-chance visual performance in a field made blind by V1 damage, in the absence of conscious visual experience.

Calcarine sulcus.

The deep horizontal fissure on the medial occipital surface along which the primary visual cortex is folded.

Cortical magnification.

The over-representation of the central visual field on cortex; the millimetres of cortex per degree fall steeply with eccentricity.

Dorsal stream.

The occipito-parietal pathway serving spatial vision and the visual guidance of action (the where/how stream).

Extrastriate cortex.

The visual cortex surrounding V1 (areas V2, V3, V4, V5/MT and beyond) where specialised processing occurs.

Fusiform face area.

A region of the ventral occipito-temporal cortex that responds selectively to faces.

Homonymous hemianopia.

Loss of the same contralateral half of the visual field in both eyes, typical of a complete unilateral V1 lesion.

Lateral geniculate nucleus.

The thalamic relay that receives retinal input and projects, via the optic radiation, to the primary visual cortex.

Macular sparing.

Preservation of central vision within an otherwise hemianopic field, attributed to the large, dually supplied foveal representation.

Ocular-dominance column.

A stripe of V1 that responds preferentially to input from one eye; adjacent stripes alternate between the eyes.

Optic radiation.

The white-matter tract carrying signals from the lateral geniculate nucleus to V1.

Orientation column.

A column of V1 neurons sharing a preferred edge orientation; preferred orientation rotates smoothly across the surface.

Population receptive field.

The aggregate region of visual field driving a voxel or population, estimated from functional imaging to map retinotopy.

Primary visual cortex (V1).

The first cortical stage of vision, in the calcarine sulcus; also called striate cortex for its myelinated stria of Gennari.

Quadrantanopia.

Loss of one quarter of the visual field, from damage to one bank of the calcarine sulcus.

Receptive field.

The region of the visual field within which a stimulus alters a neuron's firing.

Retinotopy.

The orderly mapping of visual-field position onto cortical position, preserved through many visual areas.

Scotoma.

A localised region of reduced or absent vision within the visual field.

Ventral stream.

The occipito-temporal pathway serving object recognition and identity (the what stream).

Visual field map.

A cortical area containing an orderly, typically hemifield representation of visual space; the occipital cortex holds many abutting maps.

## Key Researchers

Kalanit Grill-Spector. Characterised the functional architecture of the human ventral temporal cortex and its category-selective regions, and their role in visual categorisation. Wikipedia - Stanford profile

David H. Hubel (1926–2013). With Wiesel, mapped receptive fields and the orientation and ocular-dominance columns of the striate cortex, work recognised by the 1981 Nobel Prize in Physiology or Medicine. Wikipedia

Nancy Kanwisher. Co-discovered the fusiform face area and characterised category-selective regions of the human occipito-temporal cortex. Wikipedia - MIT profile

Brian A. Wandell. Mapped human visual field maps and retinotopy with functional imaging, author of Foundations of Vision, and a member of the National Academy of Sciences. Wikipedia - Stanford profile - Google Scholar

Torsten N. Wiesel (born 1924). Co-recipient of the 1981 Nobel Prize with Hubel for the functional architecture of the visual cortex; established the critical period for ocular dominance. Wikipedia - Rockefeller faculty

Semir Zeki (born 1940). Demonstrated functional specialisation in the extrastriate cortex — colour in V4, motion in V5/MT — and founded the field of neuroesthetics. ORCID - Wikipedia - UCL profile

## Frequently Asked Questions

What does the occipital lobe do?
It is the brain's main visual processing region. It receives retinal signals relayed through the thalamus and analyses them in a hierarchy of areas, beginning with the primary visual cortex and extending into specialised extrastriate regions for colour, motion, form and object recognition.

Where is the occipital lobe located?
At the very back of each cerebral hemisphere, behind the parietal and temporal lobes. Its primary visual cortex is folded into the calcarine sulcus on the medial surface, so much of it is hidden from a side view of the brain.

What is the primary visual cortex?
Also called V1 or striate cortex, it is the first cortical stage of vision, lining the calcarine sulcus. Its neurons respond to oriented edges within small receptive fields and are arranged in orientation and ocular-dominance columns.

What is retinotopy?
The orderly mapping of visual-field position onto cortical position: neighbouring points in the field activate neighbouring points on cortex. The central visual field is greatly over-represented, an effect called cortical magnification.

What happens if the occipital lobe is damaged?
Damage removes a region of vision whose shape reflects the lesion site. A complete unilateral lesion causes homonymous hemianopia; damage to one bank of the calcarine sulcus causes a quadrantanopia. Central vision is often spared.

What is blindsight?
The ability of some people with primary visual cortex damage to detect or localise stimuli in their blind field above chance while reporting no conscious vision. It reflects visual pathways that bypass V1.

What are the dorsal and ventral streams?
Two processing pathways leaving the occipital lobe. The dorsal stream runs toward the parietal lobe for spatial vision and action; the ventral stream runs toward the temporal lobe for object identity and recognition.

Who discovered how visual cortex neurons work?
David Hubel and Torsten Wiesel, whose single-unit recordings revealed orientation-selective neurons and the columnar architecture of the striate cortex, earning them the 1981 Nobel Prize in Physiology or Medicine.

References