Humans are highly visual animals constantly using their eyes to make decisions about the world. With forward facing eyes like other primates, we use vision to sense those many aspects of the environment that are remote from our bodies. Light is a form of electromagnetic energy that enters our eyes where it acts on photoreceptors in the retina. This triggers processes by which neural impulses are generated and then travel through the pathways and networks of the visual brain.
Separate pathways to the midbrain and the cerebral cortex mediate different visual functions – detecting and representing motion, shape, colour and other distinctive features of the visual world. Some but not all are accessible to consciousness. In the cortex, neurons in a large number of distinctive visual areas are specialised for making different kinds of visual decisions.
Light on the eye
Light enters the eye through the pupil and is focused, by the cornea and the lens, on to the retina at the back of the eye. The pupil is surrounded by a pigmented iris that can expand or contract, making the pupil larger or smaller as light levels vary. It is natural to suppose that the eye acts like a camera, forming an ‘image’ of the world, but this is a misleading metaphor in several respects. First, there is never a static image because the eyes are always moving.
Second, even if an image on the retina were to send an image into the brain, “seeing” this next image would then need another person to look at it – a person inside the brain! To avoid an infinite regression, with nothing really explained along the way, we confront the really big problem that the visual brain has to solve – how it uses coded messages from the eyes to interpret and make decisions about the visual world.
Once focused on the retina, the 125 million photoreceptors arranged across the surface of the retina respond to the light that hits them by generating tiny electrical potentials. These signals pass, via synapes through a network of cells in the retina, in turn activating retinal ganglion cells whose axons collect together to form the optic nerve. These enter the brain where they transmit action potentials to different visual regions with distinct functions.
Much has been learned about this earliest stage of visual processing. The most numerous photoreceptors, called rods, are about 1000 times more sensitive to light than the other, less numerous category called cones. Roughly speaking, you see at night with your rods but by day with your cones.
There are three types of cones, sensitive to different wavelengths of light. It is oversimplification to say it is the cones simply produce colour vision – but they are vital for it. If overexposed to one colour of light, the pigments in the cones adapt and then make a lesser contribution to our perception of colour for a short while thereafter.
The next steps in visual processing
The optic nerve of each eye projects to the brain. The fibres of each nerve meet at a structure called the optic chiasm; half of them “cross” to the other side where they join the other half from the other optic nerve that have stayed “uncrossed”. Together these bundles of fibres form the optic tracts, now containing fibres from both eyes, which now project (via a synaptic relay in a structure called the lateral geniculate nucleus) to the cerebral cortex. It is here that internal “representations” of visual space around us are created.
In a similar way to touch, the left-hand side of the visual world is in the right-hemisphere and the right-hand side in the left-hemisphere. This neural representation has inputs from each eye and so the cells in the visual areas at the back of the brain (called area V1, V2 etc.) can fire in response to an image in either eye. This is called binocularity.
The visual cortex consists of a number of areas, dealing with the various aspects of the visual world such as shape, colour, movement, distance etc. These cells are arranged in columns. An important concept about visually responsive cells is that of the receptive field- the region of retina over which the cell will respond to the prefered kind of image. In V1, the first stage of cortical processing, the neurons respond best to lines or edges in a particular orientation.
An important discovery was that all the neurons in any one column of cells fire to lines or edges of the same orientation, and the neighbouring column of cells fires best to a slightly different orientation, and so on across the surface of V1. This means cortical visual cells have an intrinsic organisation for interpreting the world, but it is not an organisation that is immutable. The extent to which an individual cell can be driven by activity in the left or right eye is modified by experience. As with all sensory systems the visual cortex displays what we call plasticity.
The intricate circuitry of the visual cortex is one of the great puzzles that has preoccupied neuroscientists. Different types of neurons are arranged across the six cortical layers, connected together in very precise local circuits that we are only now starting to understand. Some of their connections are excitatory and some inhibitory.
Certain neuroscientists have suggested there is a canonical cortical microcircuit like chips in a computer. Not everyone agrees. We now think the circuitry in one visual area has many similarities to that in another, but there could be subtle differences that reflect the different ways in which each bit of the visual brain interprets different aspects of the visual world. Study of visual illusions has also given us insight into the kind of processing that may be going on at different stages of visual analysis.
Decision and Indecision
A key function of the cerebral cortex is its ability to form and act upon sensory information received from many sources. Decision making is a critical part of this capability. This is the thinking, knowledge-based, or “cognitive” part of the process. Available sensory evidence must be weighed up and choices made (such as to act or refrain from acting) on the best evidence that can be obtained at that time. Some decisions are complex and require extended thinking while others can be simple and automatic. Even the simplest decisions involve an interplay between sensory input and existing knowledge.
Decisions about motion and colour
A subject of great current interest is how neurons are involved in making decisions about visual motion. Whether or not an object is moving, and in which direction, are critically important judgments for humans and other animals. Relative movement generally indicates that an object is different from other nearby objects.
The regions of the visual brain involved in processing motion information can be identified as distinct anatomical regions by examining the patterns of connections between brain areas, by using human brain imaging techniques and by recording the activity of individual neurons in non-human animals.
Believing is seeing
Area V5 does more than just register the motion of visual stimuli, it registers perceived motion. If visual tricks are played such that an area of dots are perceived as moving in one direction or another only by virtue of the motion of surrounding dots, i.e. an illusion of movement, the neurons corresponding to the area of the illusion will fire differently to rightwards or leftwards perceived movement.
If the movement is completely random, neurons that normally prefer rightwards movement fire slightly more on trials when the observer reports that the random motion signal is moving “rightwards” (and vice versa). The difference between neuronal decisions of “rightwards” or “leftwards” reflects what the observer judges about the appearance of motion, not the absolute nature of the moving stimulus.
Other examples of visual decision and indecision include reactions to perceptual targets that are genuinely ambiguous, such as the so-called Necker cube. With this type of stimulus the observer is placed in a state of indecision, constantly fluctuating from one interpretation to another. A similar rivalry is experienced if the left eye sees a pattern of vertical lines while the right eye sees a pattern of horizontal lines.
The resulting percept is termed binocular rivalry, as the observer reports first that the vertical lines dominate, then the horizontal lines and then back again to vertical. Once again, neurons in many different areas of the visual cortex reflect when the observer’s perception switches from horizontal to vertical.
Our visual world is an astonishing place. Light entering the eyes enables us to appreciate the world around us ranging from the simplest of objects through to works of art that dazzle and beguile us. Millions and millions of neurons are involved, with their duties ranging from the job of a retinal photoreceptor responding to a speck of light through to a neuron in area V5 that decides whether something in the visual world is moving. All of this happens apparently effortlessly within our brains. We don’t understand it all, but neuroscientists are making great strides.