What do we know about the role of primary visual cortex in vision?

The cerebral cortex is a very thin (2mm thick), highly specialised sheet of tissue wrapped around the brain. Despite this, cortical volume is very large due to the many folds or convolutions. In humans the surface area is about 2000cm2. This area is larger still in dolphins but smaller in lower mammals such as rodents and minimal in amphibians. The posterior part of the cortex is called the occipital cortex. Within this region lie several differentiable visual areas including the primary visual cortex, which can also be known as visual area 1 (V1).

Light bleaching the photo pigment in rods and cones results in the creation of a receptor potential which in turn results in a reduction in the release of the neurotransmitter glutamate. Due to this reduction in neurotransmitter, the membrane of a bipolar cell depolarises, this new potential, in turn depolarises the membrane of a ganglion cell, which increases its rate of firing. The axons of ganglion cells converge to form the optic nerve and pass visual information to the lateral geniculate nucleus (LGN). The optic nerves from the two eyes join together at the base of the brain and form the optic chiasm. At this point axons from the inner halves of the retina cross over and go up to the LGN in the opposite hemisphere. This means that both the LGN and the primary visual cortex receive information from the contra lateral half of the visual field. The LGN consists of six main layers, each receiving information from one eye. The inner two layers contain larger cell bodies and are known as the magnocellular layers. The outer four layers contain smaller cell bodies and are known as the parvocellular layers. The koniocellular sub layer is located ventrally to the other layers. This layer is responsible for transmitting information from short wavelength ('blue') cones to the primary visual cortex. In turn the cell bodies of the LGN send their axons through a pathway known as the optic radiations to V1.

V1 consists of six main layers, arranged in parallel to the surface. This cortical area is also known as the striate area as the layers consist of the nuclei of cell bodies and dendritic trees which show up as dark and light bands in stained sections of tissue. The middle layer (4C) receives information from the magnocellular and parvocellular layers of the LGN. The information is then passed to different layers to be analysed. Layers 2 and 3 receive information from the koniocellular layers, and are also responsible for passing information onto visual association areas.

Hubel and Weisel carried out pioneering work into cortical cell function, firstly on neurons in the cortexes of cats and monkeys (60s&70s). They found that small spots of light, so effective at stimulating the circular receptive fields of cells in the retina and LGN are much less effective as stimuli for cortical receptive fields. Cortical cells respond to more than just light spots, they respond selectively to features. Cortical cells are capable of analysing more abstract information than just light intensity and hue. Information in the cortex is combined in such ways that features can be detected that are larger than the receptive field of single retinal cells. Most neurons in V1 are sensitive to the orientation of a bar for example. Hubel and Weisel classified these cells into three different types. Simple cells have elongated, opponent receptive fields. For example such a cell could have an excitatory (ON) central region with inhibitory regions on either side. Each simple cell is responsive to a very small (10') range of orientations. The location of the stimulus is as important as it's orientation. Complex cells are also orientation sensitive but don't have excitatory and inhibitory regions. These cells also increase their rate of firing if the line moves perpendicular to the original position. The size of their receptive fields is larger so they can also detect movement. They also respond to light bars on dark backgrounds and equally vice versa. Hypercomplex cells are again orientation detectors but also demonstrate end inhibition; the cells increase their firing when they detect the end of a line at a particular orientation.

It appears that the cells in the cortex are somewhat hierarchical. A complex cell surveys the activity of a group of simple cells that in turn survey the cells that first enter layer 4C (stellate cells) and so on down to the retinal ganglion cells. It is these convergent connections that are thought of as the initial steps in perception. At each successive level the cells are equipped to analyse more abstract information. The cells of the primary visual cortex respond to lines and boundaries that comprise the visual fields of several lower cells.

The cells have been described as feature detectors; each cell will respond to a slightly different orientation, and will have different excitatory/inhibitory regions. For example simple or complex cells 'seeing' the sides and hypercomplex cells seeing the corners would detect a triangle. This seems intuitively possible but problems are encountered when the idea is extended; are their feature detectors for every object in the world? There are not enough cortical cells for them to be this specific, what if a neuron died, would we lose the ability to recognise a certain object? Each cortical cell responds to a variety of stimuli for example, bars, edges, waves, checkerboard patterns, and so on.

Albrecht et al. (1978) found that cortical cells are also 'spatial frequency analysers'. They discovered that neurons in V1 responded better to sine wave gratings than to lines or edges. A sine wave grating is designated by its spatial frequency (its variation in brightness measured in cycles per degree of visual angle). Different neurons appear to detect different spatial frequencies (at different orientations). It appears that most receptive fields are large enough to include between 1.5 and 3.5 cycles of the grating. Spatial frequency is a valid way of analysing information as, for example, small objects and large objects with sharp edges provide high spatial frequency signals whilst low frequencies are signalled from large areas of light and dark. The most important visual information comes from low spatial frequencies, as when such information is removed, shapes are very hard to perceive. On the other hand removing high spatial frequency information simply reduces the detail, or visual acuity. Harmon and Julesz (73) constructed a picture of Abraham Lincoln on a computer using small squares representing the average brightness in specific portions of the picture. A second picture was created by removing the high spatial frequencies from the original image, i.e. all the sharp lines (visual noise). When the visual association cortex receives such noisy information (the first picture) it finds it hard to perceive the underlying image.

In 1992 von der Heydt et al. found a new type of cortical cells whilst studying monkey striate cortex. These neurons appear to respond to 'periodic patterns'. They don't respond to lines or bars, rather to sine wave gratings of particular spatial frequency, orientation and frequency. Von der Heydt claimed that there are approximately 4 million cells of this type serving just the central 4 degrees of vision in monkey striate cortex. This type of cell is clearly capable of analysing more complex information than cells lower in the visual pathway, such as ganglion cells that can detect sudden changes in brightness.

A further difference in LGN and cortical cells is that the cortex contains binocular cells. They receive information from both eyes. V1 is the first point in the visual pathway to combine information from both eyes. A function of the primary visual cortex is therefore depth perception involving retinal disparities. Many of these binocular cells especially in layer 4C (from the magnocellular layers) have response patterns that seem to aid the perception of depth. These cells respond most strongly when the same object is seen in slightly different locations by the two eyes. That is they respond to retinal disparity (images falling on slightly different parts of the retina), which is used in stereoscopic vision, indicating differences in the distances of objects. It is suggested that it is the horizontal disparities of these binocular neurons that is the neural basis for stereo depth. It is possible that having a collection of neurons each responding to a single disparity creates this binocular response. The disparity could be estimated by identifying the neuron with the largest response. Richards (1971) suggested an alternative; disparity might be measured by creating a few pools of neurons with coarse disparity tuning. The pools might respond to objects in slightly different positions relative to the horopter (arc swept about the viewer where any object on the arc will fall on corresponding points on the retinas of the two eyes). To estimate depth the relative responses from the pools could be compared.

One last main function of the primary visual cortex is colour perception. Colour sensitive ganglion cells pass information through the parvocellular and koniocellular layers of the LGN to the primary visual cortex. This information is transmitted to special cells in cytochrome oxidase (CO) blobs. These specialised areas of the striate cortex were discovered by Wong-Riley in 1978 when he found that a stain for cytochrome oxidase (an enzyme in mitochondria) showed a patchy distribution in the cortex. Later research with the same stain (e.g. Horton and Hubel 1980) showed a polka dot pattern of dark columns through layers 2 and 3 and (to a lesser extent) layers 5 and 6. It has recently been discovered that the parvocellular system receives and transmits information about the long and middle wavelengths ('red' and 'green' cones) whilst the koniocellular system does the same for short wavelengths ('blue' cones). The blob cells are double opponent, meaning a single cell can receive input from more than one type of cone cell. For example some cells have centres that respond in and excitatory way to 'red' cones and a surround that responds in an inhibitory way to the same cones. In the same cells 'green' cones have the opposite effect; inhibit in the centre and excite in the surround. Cells in the blobs have circular receptive fields and are therefore not responsive to orientation. From V1 the parvocellular-blob system projects to thin strips in V2 and then onto V4 where there are colour selective cells. In this way the colour of a stimulus is processed separately from form and movement until the visual association cortex.

It is mostly agreed that the cortex is organised into modules. Each module consists of two segments each surrounding a CO blob. Blob neurons are sensitive to colour and low spatial frequencies but relatively insensitive to other features for example orientation. Neurons outside the blobs seem to be the opposite, they are mostly insensitive to colour but do show sensitivity to orientation, movement, spatial frequency, texture and binocular disparity. The two segments receive information from different eyes but the module then combines the information.

The receptive fields of neurons in the same module overlap, demonstrating that they receive information from the same area of the visual field. By inserting a microelectrode straight into an interblob region, it has been found that both simple and complex cells are present but they respond to the same orientation and share the same ocular dominance (same percentage of input from the two eyes). Orientation sensitivity and ocular dominance vary systematically around the interblob region and are arranged at right angles to each other. Cells are arranged in columns about 0.5mm across according to their 'orientation preference'. The connection between simple and complex cells in the same module allow for the first deconstruction of the visual world into lines of various orientations, this is a necessary requirement for the visual analysis of form.

Blasdel (1992) invented an inspired way of mapping the modules in the striate cortex. Monkeys were operated on to have part of their skull removed and replaced with a glass window. This window was made so that it allowed voltage sensitive dye to be injected into the cortex. The dye was injected and the monkey's shown different visual stimuli designed to cause responses in neurons sensitive to certain features.

An interesting phenomenon found when the visual cortex is damaged is blindsight. Damage to earlier stages in the visual pathway results in blindness in all or part of the visual field. People with blindsight do not see anything in the blind part of their visual field and yet they can point at or reach for objects located in this region and can discriminate their size and orientation. This behaviour appears to occur with out any conscious awareness of the objects.

The primary visual cortex is an important stage in visual perception but is not responsible for all the processing. Cortical neurons can only analyse parts of the visual field at any one time. It is not until the visual association cortex that these parts are combined. The modules in the striate cortex respond selectively to colour, orientation, movement, spatial frequency, texture and retinal disparity to build up the image of the visual field. This information is then passed to the visual association cortex, for example colour processing occurs in the CO blobs (after the P and K systems) and is then transmitted to areas V4 and V8 of the extrastriate cortex. Damage to these areas abolishes colour constancy and causes loss of colour vision respectively. The primary visual cortex is responsible for further analysis of form, movement and colour in larger areas of the visual field than have been looked at in the retina and LGN. Form and movement appear to follow a different pathway to the analysis of colour. The two pathways were also separated in the LGN in the different layers. This information is combined in the visual association cortex.