Saturday, January 06, 2007

Watching the brain make up its mind about an ambiguous stimulus.

On looking at an ambiguous visual stimuli, we can experience frequent spontaneous transitions between two competing percepts while physical stimulation remains unchanged (see example in the top section of Figure 1)... a key question has remained unresolved: Does perceptual rivalry result merely from local bistability of neural activity patterns in sensory stimulus representations (i.e. mainly in posterior visual cortical areas), or do higher-order areas (i.e. frontal cortex) play a causal role by shifting inference and, thus, initiating perceptual changes? Sterzer and Kleinschmidt have used functional MRI to measure brain activity while human observers reported successive spontaneous changes in perceived direction for an ambiguous apparent motion stimulus (Fig 1, top). In a control condition, the individual sequences of spontaneous perceptual switches during bistability were replayed by using a disambiguated version of the stimulus (Fig. 1, bottom).

Fig. 1. Stimulus display. Ambiguous and disambiguated versions of the apparent motion quartet used in the rivalry and replay conditions, respectively, are shown. The single frames alternated at 4 Hz. When looking at the rivalry stimulus, perception is bistable and fluctuates spontaneously between periods of horizontal and vertical apparent motion perception. Disambiguated versions of the stimulus were used to change participants' perception of apparent motion with the same temporal sequence as during the rivalry condition.

Fig. 2. Transient activation during perceptual switches. (A) Regions commonly activated in response to both spontaneous and stimulus-driven perceptual switches are rendered in blue onto a standard anatomical template image Numbers 1–6 indicate the regions that were subsequently used for detailed analyses of signal time courses. (B) Greater response amplitudes during spontaneous as opposed to stimulus-driven switches were observed in bilateral inferior frontal regions and are shown in red. (C) Earlier responses during spontaneous as opposed to stimulus-driven switches were observed in the right inferior frontal gyrus.

Greater activations during spontaneous compared with stimulus-driven switches were observed in inferior frontal cortex bilaterally. Subsequent chronometric analyses of event-related signal time courses showed that, relative to activations in motion-sensitive extrastriate visual cortex, right inferior frontal cortex activation occurred earlier during spontaneous than during stimulus-driven perceptual changes. The temporal precedence of right inferior frontal activations suggests that this region participates in initiating spontaneous switches in perception during constant physical stimulation. Their findings can thus be seen as a signature of when and where the brain "makes up its mind" about competing perceptual interpretations of a given sensory input pattern.

No comments:

Post a Comment