Neuronal mechanisms for the perception of ambiguous stimuli

Ambiguous figures that may take on the appearance of two or more distinct forms have fascinated philosophers and psychologists for generations. Recently, several laboratories have studied the neuronal basis of perceptual appearance at the level of single neurons in the cerebral cortex. Experiments that integrate neuronal recording with analyses based on sensory detection theory reveal a remarkable degree of specificity in these neuronal responses. The new challenges are to understand how cognitive processes, such as attention and memory, interact with perception to generate these neuronal signals.     

United we sense, divided we fail: context-driven perception of ambiguous visual stimuli

Ambiguous visual stimuli provide the brain with sensory information that contains conflicting evidence for multiple mutually exclusive interpretations. Two distinct aspects of the phenomenological experience associated with viewing ambiguous visual stimuli are the apparent stability of perception whenever one perceptual interpretation is dominant, and the instability of perception that causes perceptual dominance to alternate between perceptual interpretations upon extended viewing. This review summarizes several ways in which contextual information can help the brain resolve visual ambiguities and construct temporarily stable perceptual experiences. Temporal context through prior stimulation or internal brain states brought about by feedback from higher cortical processing levels may alter the response characteristics of specific neurons involved in rivalry resolution. Furthermore, spatial or crossmodal context may strengthen the neuronal representation of one of the possible perceptual interpretations and consequently bias the rivalry process towards it. We suggest that contextual influences on perceptual choices with ambiguous visual stimuli can be highly informative about the neuronal mechanisms of context-driven inference in the general processes of perceptual decision-making.     

Hemispheric interaction in man during perception of visual stimuli]

Averaged evoked potentials (AEP) to verbal (letters) and nonverbal (random shapes) stimuli exposed in the left and right visual fields were registered in healthy subjects with normal vision. Analysis of the later AEP latencies pointed to asymmetry in the temporal parameters of the interhemispheric interaction. The late AEP latency is shorter in the right hemisphere than in the left hemisphere. The difference is more pronounced in responses to nonverbal stimuli. The earlier development of the evoked potential in the right hemisphere (or the later one in the left hemisphere) accounts for the interhemispheric difference in the temporal parameters of the late AEP components. Comparison of the latency of the component P300 to verbal and nonverbal stimuli presented in the ipsilateral or the contralateral visual fields reveals a transfer of the results of the cortical processing of visual information in the course of interhemispheric interaction.     

Reaction time to visual stimuli in child development

The simple and differential reaction time and time of cognitive processes were studied in 3-7-year-old children using age-adapted computer technique. The reaction time significantly decreased with age in parallel with improvement of cognitive processes. An experimental method is proposed, which makes it possible to determine what kind of cognitive process is responsible for age-related decrease in the reaction time.     

Perceptual dominance time distributions in multistable visual perception

Perceptual multistability, alternative perceptions of an unchanging stimulus, gives important clues to neural dynamics. The present study examined 56 perceptual dominance time series for a Necker cube stimulus, for ambiguous motion, and for binocular rivalry. We made histograms of the perceptual dominance times, based on from 307 to 2478 responses per time series (median=612), and compared these histograms to gamma, lognormal and Weibull fitted distributions using the Kolmogorov–Smirnov goodness-of-fit test. In 40 of the 56 tested cases a lognormal distribution provided an acceptable fit to the histogram (in 24 cases it was the only fit). In 16 cases a gamma distribution, and in 11 cases a Weibull distribution, were acceptable but never as the only fit in either case. Any of the three distributions were acceptable in three cases and none provided acceptable fits in 12 cases. Considering only the 16 cases in which a lognormal distribution was rejected (p<0.05) revealed that minor adjustments to the fourth-moment term of the lognormal characteristic function restored good fits. These findings suggest that random fractal theory might provide insight into the underlying mechanisms of multistable perceptions.     

A theory of ambiguous pattern perception

A theory of ambiguous pattern perception is formulated. This theory proposes a feature selector (field of attention) based on the time-sequential discrete property of the attention, a short-term memory for storage of the selected features, and a displayer (perception) to display the consecutively stored features. Since the selected features continuously enter, and since the features can only be stored in the short-term memory for a short period, the features which can be displayed in the displayer vary with time. When all the essential features belonging to one pattern happen to be in the displayer, the picture is perceived to be that pattern; when all the essential features belonging to another pattern happen to be in the displayer, then the picture is perceived to be the other pattern. Thus the picture appears to vary with time and alternate between two patterns. A numerical calculation is presented.     

Manifestations of the genotypic causality of human evoked potentials during perception of various visual stimuli

Intrapair resemblance of the wave form and amplitude-temporal parameters of evoked potentials (EPs) to flashes, chess field, house image, the word "house" and a series of other stimuli was evaluated in 20 pairs of monozygotic and 20 pairs of homosexual dizygotic adult twins. In the occipital area the maximum of genetic dependence was characteristic of EPs to flashes, the minimum--of EPs--to the word "house". In vertex EPs parameters genotypic effects were manifest irrespectively of the stimulus type. Genotypic dependence differed for the amplitudes and latencies of separate EP components.     

Bioelectric brain activity during visual perception of verbalized and nonverbalized stimuli in seven-to eight-year-old schoolchildren

Neurophysiological mechanisms of visual perception of verbalized and nonverbalized stimuli have been studied in girls (n = 89) and boys (n = 109) aged seven to eight years. It has been shown that, in children of this age, neurophysiological mechanisms of the visual perception system during the selection of verbalized and nonverbalized stimuli have similar organizations of functional rearrangements and are characterized by the general involvement of cortical areas and by the lack of clear specialization of the cerebral hemispheres. However, the choice of nonverbalized visual stimuli is characterized by a more pronounced generalization of the functional interaction between cortical areas and by the increased spectral power of the EEG’s high-frequency components in anterior associative areas. The increased spatial synchronization in the EEG θ- and β bands in the frontal areas emphasizes the contribution of these structures to the analysis and processing of visual stimuli at this stage of development. Differences in the neurophysiological mechanisms of visual perception in boys and girls have been detected. Spatial and temporal organizations of bioelectric activity (BEA) of the brain in girls performing visual tasks is characterized by a stronger and more varied ipsi- and contralateral statistical relationships in the fronto-occipital direction and displacement of interaction centers to the left hemisphere. In boys perceiving verbalized and nonverbalized visual stimuli, a greater contribution of posterocentral cortical areas has been observed. The detected differences are considered to reflect a greater maturity of identification mechanisms in girls aged seven to eight years as compared to boys.     

Effect of genotype on evoked potentials of the left and right hemispheres during perception of visual stimuli

Intrapair similarity of evoked potentials (EPs) to light flash, checkerboard, house picture, the word "house" and a number of other stimuli was estimated in 20 pairs of monozygotic and 20 pairs of dizygotic adult twins of the same gender. In the right temporal area the level of the genotypic control of amplitude-temporal parameters of EPs on the whole is higher than in the left one; the more prominent this difference is in responses to spatial-structural stimuli. In the left temporal area, the genotypic control is manifested in latencies of components of EPs to linguistic stimuli.     

Unconscious context control of visual perception of simple stimuli: A study using evoked potentials

The effect of nonsemantic context on the perception of simple nonverbal visual stimuli has been studied in ten healthy volunteers by the event-related potential (ERP) method. The nonsemantic context was specified by the formation of a memory trace of a test visual stimulus via its repeated presentation without any instruction except gaze fixation. Then, this stimulus randomly alternated with control stimuli that did not form memory traces before their presentation. It has been found that an ERP in the interval 260–340 ms after presentation of a simple nonverbal stimulus significantly differs from the control ERPs. The results suggest that some stages of the processing of visual stimuli may be modified by nonsemantic context.     

Reversal of signal significance of stimuli in the avoidance reflex]

The course of reversal learning in go -- no go avoidance reflex differentiation in both cats and dogs was analysed in terms of interrelations between drive and instrumental reflex activity. When signalling properties of the conditioned stimuli were reversed, marked changes in the drive state of experimental animals occurred. The increase of the fear drive influenced both the transfer of the instrumental response to the new positive conditioned stimulus and the extinction of the response to the previously positive stimulus. The quality of the conditioned stimuli and their reflexogenic strength exerted clear effects on the fear drive and on the course of reversal learning. Prefrontal lesions affected the drive state and the conditioned reflex activity of the animals during the reversal learning.     

Oscillations in the perception of ambiguous patterns a model based on synergetics

An algorithm which can be realized by synergetic systems and which was introduced by one of us (Haken 1987) for the recognition of patterns is extended so that the perception of ambiguous patterns can be modelled. In this approach so-called attention parameters are subjected to a damping mechanism mimicking the effect of saturation of attention. In this way oscillations of perception arise quite naturally. Our approach takes also ambiguous patterns with bias into account which leads to different periods of the attention paid to the one or the other interpretation of the pattern. Our results are in good agreement with previous psycho-physical studies by other authors. Finally we show how hysteresis of perception can be modelled.