Sensory, Attentional, and Perceptual Processes- 2 - Free MCQ Practice Test

Some further examples of perceptual phenomena that result from the process of exaggeration are shown in the ‘Everyday Psychology’ box . These are known collectively as simultaneous contrast illusions. In each case the central regions of the stimuli are identical, but their surrounds differ. Panel A lets you experience the simultaneous tilt illusion, in which vertical stripes appear tilted away from the tilt of their surrounding stripes. Panel B shows the luminance illusion: a grey patch appears lighter when surrounded by a dark area than when surrounded by a light area. Panel C shows the same effect for colour: a purple patch appears slightly closer to blue when surrounded by red, and closer to red when seen against a blue background. There is also an exactly analogous effect for motion, as well as for other visual dimensions such as size and depth.

The recurrent processing model emphasizes the effects of a stimulus on the higher centres of the brain, suggesting that subjective perception and actual perception at the level of the sensory receptors is modulated by these higher processes. Serial processing and parallel processing were earlier models of perceptual processing, with the parallel model having some support but the serial model is completely outdated. Selective adaptation is not a model of visual processing, but rather a technique used by the sensory system to adjust to the properties of a constant background. Feedback processing is not a term used to refer to a visual processing model.

This system of channels can signal orientations which do not correspond to the preferred orientation of any single channel. How is the information from all these channels combined when a visual stimulus is presented? There is likely to be a process that combines the activities across all channels, weighted according to the level of activity in each channel. Such a process finds the ‘centre of gravity’ of the distribution of activity. The centre of gravity (in statistical terms, the weighted mean) corresponds to the perceived orientation of the stimulus.

Parallel search tasks can be contrasted with a second search type, conjunction search, in which the target/distractor difference is not based on a single feature, but on conjunctions of features. For example, the target might be a vertical red line in an array of vertical blue lines and tilted red lines. In this scenario, search time for the target is not constant, but instead rises with the number of distractors. The observer apparently searches through the display serially, scanning each item (or small group of items) successively (serial search).
Both positive and negative trials are presented. In positive trials the target is present in the display, whereas in negative trials the target is absent in the display. This kind of task might arise in real life when you have forgotten the location of your car in a large car park. You have to find a blue Ford amongst an array of cars of many makes and colours, where, for example, red Fords and blue Volkswagens are the distractors. The target does not pop out, but finding it requires effortful attentive scrutiny (Treisman & Gormican, 1988).

To find one’s car in this situation would require a conjunction and serial search because it is necessary to search for the type and colour of car (conjunction of features) and to scan all the cars in order to be sure the target car (one’s own car) is not missed. Parallel search is used to find a target independent of the number of items or distracters in the visual display.

Cortical cells are vertically organized in columns which stretch the entire depth of the cerebral cortex and respond to similar properties of the stimulus, while neighbouring columns respond to different aspects of the stimulus or visual field. Cells are not arranged horizontally, in straight lines, by volume, or around the optic nerve.

The theory of parallel cortical organization became complicated by Livingstone and Hubel’s description of activity in a third type of column in V2, where the cells receive converging input from the magno and parvo systems. They suggested that these columns are used for spatial pattern analysis. However there are problems with this scheme. For example, Livingstone and Hubel claimed that images in which the different regions are red and green, but all of the same brightness, appear flat. They attributed this to the insensitivity of cells in the magno/depth system to differences purely in hue, which are detected primarily by the parvo system. Quantitative studies, however, found that perceived depth is not reduced at all in such images (Troscianko et al., 1991). It appears, then, that depth percepts can be derived from both magno and parvo information, though not necessarily equally well at all distances (Tyler, 1990).

Vernier acuity is the term used to refer to the ability to see very small differences in the alignment of two objects, especially when the objects are close to one another. The other terms given do not refer to this specific ability.

Top-down processing refers to the influence of higher brain centres and processes on the perception of stimuli and not solely the product of sensory input. Bottom-up refers to the sensory input only. Vertical, horizontal, and parallel are not relevant terms to this specific process.

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 is offset to the left or right of the upper line (a vernier acuity task).
Humans can discern the direction of very tiny offsets, but can improve even more with practice, though this may require thousands of presentations (Fahle & Edelman, 1993).
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 part of the brain where the temporal, parietal and occipital lobes of the brain join) may be able to perform normally on feature search tasks but are markedly impaired in conjunction search tasks (Arguin et al., 1993).
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 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.