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What can the study of after effects tell us about how the brain processes visual information?

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Introduction

What can the study of after effects tell us about how the brain processes visual information? Motion and orientation are two important features of the visual world. Psychophysical evidence exists to demonstrate that common computational principles underlie the processing of these two visual modalities. Two well-known after effects, the motion after effect (MAE) and the tilt after effect (TAE) will be described to explain how the brain processes visual information. After effects occur when one of our senses is exposed for a period of time to an unchanging stimulus then our subsequent perception, or our capacity to detect it will be briefly altered. Previously experiments on animals have shown that the visual system contains individual nerve cells that respond only to edge of a given orientation or movement in a particular direction and there is every reason to suppose human sensory systems are organised in the same way. (Mollon, 1974). Psychophysical studies involved adaptation is the most commonly used technique. There are two ways information is transmitted in sensory systems (Mollon, 1974); coding by place and frequency coding. Coding by place is when different values on a stimulus dimension are represented by which set of neurones is active. Different neurones may correspond to different spatial frequencies. ...read more.

Middle

overcame this problem using the pressure blind technique. They found interocular transfer even when the adapted eye was pressure-blinded between looking at the adapting motion with one eye and examining for its after effect in the other. The MAE therefore did not reside in activity in the retinal ganglion cells. Wohlegemuth (1911; as cited in Antis & Duncan, 1983) found MAE's that lie in monoculary driven channels. He found that if one looks at a spiral with one eye, and at another spiral rotating in the opposite direction with the other eye this causes independent MAE's from each eye separately but no MAE is seen when both eyes are open. MAE's can result from the adaptation of either monocular or binocular neurons (Moulden, 1980; as cited in Antis & Duncan, 1983). The fact that MAE's can be mediated by monocular or binocularly driven neurons indicates MAE's are the result of two separate pathways and that motion perception occurs in two parallel systems. Dichoptic MAE's (caused when each eye is exposed to opposite directions of motion, either simultaneously or alternatively, the direction of the MAE is contingent on which eye views the stationary display). Dichoptic MAE's are said to be caused by monoculary driven neurons. Alternatively MAE's driven by binocular neurons show interocular transfer. ...read more.

Conclusion

(Kohler & Wallach, 1944) Alternatively, the TAE results from prolonged inhibition of orientation selective cells activated by the adaptation lines (Deutcsh, 1964 as cited in Paradiso) Paradiso et al (1989) used the TAE to study interactions between real and subjective contours. Participants adapted to either real of illusory lines and were then shown test stimuli containing real or illusory lines. They found that interocular transfer of the TAE was greatest when the test stimulus was subjective than when it is real. They found that the perception of subjective contours was consistent with properties of the V2 neurons which known to be orientation selective and highly binocular. The TAE, unlike the MAE shows complete interocular transfer (Campbell & Maffei, 1971). The channels that is responsible for orientation and spatial frequency are only partially transferred from one eye to the other (Blakemore & Campbell, 1969; as cited in Campbell & Maffei, 1971). From the study of after effects we have learnt there are orientation selective cells in the visual cortex, these cells may be simple or complex. The prevailing theory for the TAE is based on lateral inhibitory interactions between orientation detectors in the primary visual cortex. The way in which cells respond to visual information is the key to how the brain processes visual information. ...read more.

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