PERCEPTUAL LEARNING

HOW TRAINING INFLUENCES PERFORMANCE
Although the role of knowledge and assumptions in perception is now quite clear, the detailed ways in which past experience influences perception are less clear. Recently, experimenters have begun to examine these questions by studying how training can influence performance on apparently simple visual tasks, such as judging whether the lower line in figure 8.21 is offset to the left or right of the upper line (a vernier acuity task). [vernier acuity the ability to see very small differences in the alignment of two objects, which becomes particularly obvious when the objects are close to one another] Humans can discern the but can improve even more with practice, though this may require thousands of presentations (Fahle & Edelman, 1993). The nature of the learning can be studied by measuring the extent to which it transfers from the training stimulus to other stimuli and conditions. Thus if, after training, the vernier stimulus is rotated through 90 degrees, performance on the new task is no better than it was at the start of the experiment. Similarly, performance falls if observers are trained on one retinal location and tested on others, or trained using one eye and tested on the other.

Findings like these suggest that some of the training occurs at a site where the neurons are driven by one eye, receive input from restricted regions of the retina, and are orientation-specific. Fahle (1994) speculated that the learning might reflect changes occurring in orientation-specific neurons in V1, some of which are monocular (driven by only one eye). Others have questioned the extent and nature of the specificity of learning, and suggested that there might be a general as well as a stimulus-specific component to the observed learning effects (Beard et al., 1995). This general component might reflect, for example, a change in the ability to direct attention to particular regions of the visual field. This idea receives further support from studies into visual search conducted by Ellison and Walsh (1998). Different types of visual search not only have different behavioural characteristics, but also depend on different brain regions. So some patients with attention deficits (due to damage to the par of the brain where the temporal, parietal and occipital lobes of the brain join) may be able to perform normally on feature search [feature search visual search for a unique feature such as a particular colour or orientation (e.g. a red spot) in an array of distractors defined by different features along the same visual dimension (e.g. green spots)] tasks but are markedly impaired in conjunction search tasks (Arguin et al., 1993). Also, Ashbridge et al. (1997) used transcranial magnetic stimulation (TMS) to study the role of direction of very tiny offsets, different brain regions in visual search. In this technique, a strong magnetic field is applied briefly to the surface of a localized region of the skull, temporarily disrupting neural activity in the underlying brain region. These researchers found that stimulation of the right parietal lobe did not affect initially parallel searches, but did affect initially serial searches. Moreover, a related study found that right parietal stimulation did not affect initially serial searches once they had become parallel through training. But when the observers were switched to another task, which they initially had to perform serially, right parietal stimulation could disrupt search again (Walsh et al., 1998). Walsh et al. (1998) suggest that the right parietal lobe may be involved in setting up new templates [template an internally stored representation of an object or event in the outside world, which must be matched with the pattern of stimulation of the sensory systems before identification, recognition or naming of that object or event can occur] in the temporal lobe for processing conjunctions of, say, colour and form. Once the learning is complete, the right parietal lobe no longer plays a role in the task and so stimulating this region no longer impairs performance.