Localization and inter-ocular transfer

Other characteristics of the underlying mechanisms can also be inferred from the properties of after-effects. For example, visual after-effects are usually confined to the adapted region of the visual field. So staring at a small red patch does not change the perceived colour of the whole visual field but only of a local region. In addition, most visual after-effects show inter-ocular transfer. [inter-ocular transfer the adaptation or learning that occurs when a training stimulus is inspected with one eye and a test stimulus is subsequently inspected with the other eye] This means that if the observer stares at a stimulus with only one eye, the tilt and other after-effects can be experienced not only with the adapted eye but also with the corresponding retinal region in the other eye, which is not adapted. These two properties suggest that such after-effects are mediated by mechanisms that are linked to a particular region of the visual field and can be accessed by both eyes. In other words, they suggest that the mechanisms underlying these after-effects are located centrally (i.e. within the brain) after information conveyed from the two eyes has converged, rather than peripherally (i.e. within each eye or monocular pathway). Neurophysiologists recording the electrical activity in single nerve cells in the visual systems of cats and monkeys have discovered that in area V1 (the cortical area where information from the eyes first arrives – see figure 8.10, below), many neurons have properties that would enable them to mediate visual after-effects. Different neurons in V1 respond to the orientation, size, direction of motion, colour and distance from the animal of simple stimuli such as bars or gratings. Many of the neurons in V1 are binocular, meaning their activity can be changed by stimuli presented to either eye. And they are linked to particular and corresponding places on each retina, which means that a stimulus has to fall within a particular region (receptive field) on one or both retinas to affect them. Neurons in V1 also, as you would expect in a mechanism which mediates the tilt after-effect, adapt to visual stimulation, so their response to a stimulus declines over time with repeated presentation (Maffei et al., 1973). The localized receptive fields and binocular characteristics of these neurons correlate very well with the perceptual characteristics of after-effects described above. Although adaptation occurs in other visual cortical areas, the neurons in area V1 are prime candidates for the mechanisms that underlie visual after-effects in people. One implication of this account of early visual processing is that the images of complex objects (trees, houses, people) are initially analysed by mechanisms that respond to their local physical characteristics and have no connection with the identity of the objects themselves. From the point of view of a neuron in V1, the vertical blue edge moving to the left might as well belong to a train as to the shirt of the frustrated passenger who has just missed it and is running along the platform in desperation after it. In other words, the visual system appears initially to decompose the scene into its constituent parts and to analyze these separately (i.e. in parallel).