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Zusammenfassung:
Although objects can be identified based solely on the information provided by the spatial structure of an
image, color adds another perceptual dimension which may facilitate object identification. However, the
addition of color in form of colored noise may interfere with object identification. Here we investigate
whether color in natural images is associated with changes in visual memory performance. In addition, we
examined neural activity in area V4 and dorsolateral prefrontal cortex (PF) and address whether alterations in
neural responses are correlated with changes in performance. Additionally we explore whether there is neural
evidence of a functional specialization for color processing in natural scenes in either of the two brain areas.
We used a procedure based on Fourier analysis to degrade a unique set of four colored and achromatic natural
scenes with increasing amounts of four unique achromatic noise patterns. We also degraded achromatic
images with chromatic noise patterns. At a given degradation level, the difference between colored and
achromatic images was thus provided only by color: the remaining image specific color or the colored noise.
In a delayed matching to sample paradigm a sample stimulus (250ms) at various degradation levels was
presented, followed by an undegraded probe stimulus (1s) after a delay period (1500ms). A lever press was
required if the sample stimulus matched the probe. We have preliminary results from one monkey. The
monkey's recognition performance decreased as a function of noise for all conditions. In addition, we found
that the recognition performance was best for the 'natural' color condition and worst for the colored noise
condition at the same degradation level (paired t-tests, p<0.01). These results suggest that color independent
of spatial composition, confers either an advantage or impairment in object identification depending on
whether the color is related to the object or not.
Single neuron responses were recorded from a total of 84 neurons in V4 and 76 neurons in PF. In V4 73 of
visually responsive cells conveyed less information about image identity with increasing amounts of noise
(39/53 units, Signtest p<0.0001). In addition, at the same degradation level, the information about image
identity was significantly higher for the natural color condition than the colored noise condition (paired ttest,
p<0.05). In PF 72 of visually responsive neurons conveyed less information about image identity with
increasing amounts of noise (42/58 units, Signtest p<0.001). In PF no significant differences between the
achromatic and chromatic stimulus conditions were found. For both areas, the effect of noise on information
conveyed by neural activity about image identity is consistent with the effect of noise on behavioral
performance. Furthermore, in V4 the elevated information about image identity for image specific color vs.
colored noise is correlated with increased recognition performance in the image specific color condition.
These results reveal noise and color specific modulations in neural activity at the level of single neurons in
area V4 and PF in object identification.
