Psychophysical methodologies are commonly used to explore the relationship between the observer's psychological states, assessed via their responses in a simple task, to finely controlled manipulations of the physical stimulus.

Stimuli are carefully chosen to target only the specific perceptual processes of interest. For example, stimuli composed of rotating cylinders may reveal how the brain maintains a coherent interpretation of the scene as a whole even when the individual parts are ambiguous (see Figure 1). Other typical stimuli include configurations of striped patches, for investigating how the brain integrates local oriented stimuli into the perception of a continuous contour (Figure 2). The tasks often push observers' perceptual abilities to their limits to find the threshold at which a stimulus becomes just detectable or just discriminable from others. Different thresholds for different stimuli can help to reveal why some stimuli are easier or harder to perceive than others (see Figure 3). Such results can inspire new computational and neural models of perceptual processing in the brain, and further test their predictions.

Psychophysical experiments can be run using only a computer and a good display (plus an eye-tracker and specialized graphics generator if necessary), and important insights into brain function can be obtained just by testing small groups of normal healthy adults. However, the power is enhanced in combination with physiological measures such as EEG and fMRI, to observe how the activity of specific areas in the brain is correlated with observers' performance as a function of the stimulation or attentional instruction.

Figure 1: Ambiguous cylinders appear to rotate in synchrony

Figure 2 : Stimuli used to investigate how the brain integrates small oriented elements into the perception of a continuous contour

Figure 3: The middle patch of the three is often easier to detect when the surrounding patches are collinear (top left) versus orthogonal (top right). Bottom: EEG correlate of perceiving collinearity


Figure 4: fMRI activity associated with selectively attending to the left or right of a visual display


This page last modified 17 November, 2011 by [ICN Web Team]


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