Figure legends
Fig. 1 : (A) Lateral view of the ferret cerebral cortex displaying the rostral posterior parietal cortex (PPr). The box shows the enlarged PPr area below, with the large dots representing the location of recording penetrations (not all displayed due to overlap). (B) The experiment involved multiple single-unit recordings of responses evoked by the presentation of a visual (white bar moved nasal-to-temporal across a screen), a tactile stimulus (1 g calibrated fiber moved across skin/hairs by an electronically controlled actuator), or the combination of both visual and tactile stimulation. (C) Examples of single-unit neuronal responses recorded after visual alone, tactile alone and combined visual- tactile stimulation. Top row shows the responses of a unimodal visual neuron (left) and a unimodal tactile neuron (right). The middle row depicts a bimodal PPr neuron that generated multisensory enhancement (left) (combined response significantly greater than most effective unimodal response) and another bimodal neuron (right) that generated multisensory response depression. Bottom row shows a subthreshold multisensory response where a combined stimulus significantly enhances (left) or depresses (right) activity with a stimulus from a different modality that is not effective when presented alone.
Fig. 2. Sensory and multisensory neurons in PPr show proportional changes from infancy and across adolescence. (A) The histograms indicate the average (± SE) proportion of a particular neuron response type (V=unimodal visual; T=unimodal tactile; S=Subthreshold multisensory; B=bimodal multisensory) during the different developmental epochs (yellow=infancy; blue=early adolescence; green=middle adolescence; red=late adolescence; see Methods for definitions). (B) The same data as in part ‘A’ except now plotted with lines connecting the different response types across development.
Fig. 3 : Responsiveness (average number of spikes/response ± SE) of PPr neuron types (Unimodal =unisensory visual and unisensory tactile; Bimodal multisensory; Subthreshold multisensory) across developmental stages (Infancy=yellow; early adolescence=blue; middle adolescence=green; late adolescence- red). For subthreshold neurons, only one subthreshold such neuron was identified in infancy. Univariate ANOVA followed by posthoc Bonferroni tests * = p<0.05; ** = p<0.01.
Fig. 4 : Spontaneous activity (spikes/sec ± SE; recorded 500 ms before sensory stimulation) in unimodal (unisensory visual and unisensory tactile), bimodal multisensory and subthreshold multisensory neurons across the developmental periods (Infancy=yellow; early adolescence=blue; middle adolescence=green; late adolescence=red). For the subthreshold category, only one neuron was identified in infancy. Univariate ANOVA followed by posthoc Bonferroni tests* = p<0.05; ** = p<0.01.
Fig. 5: A. Responses of PPr neurons to multisensory stimulation across the developmental stages. (A) Pie charts indicate the p roportions of bimodal neurons that exhibited significant multisensory integration in the different age groups. B . For bimodal neurons that demonstrate MSI, these box/whisker plots show the significant increase in the magnitude of MS enhancement. No enhancement was seen during Infancy; the Early adolescence group was significantly different from Mid and Late adolescence ones. KW followed by Post Hoc Mann Whitney tests, p<0.01 in both comparisons. Mid and Late adolescence groups were not different from each other. C. For bimodal neurons that failed to demonstrate significant MSI, these box/whisker plots show the significant increase in response levels across the developmental stages. KW followed by Post Hoc Mann Whitney tests, * = p<0.05. D. Cumulative probability plots of response changes after combined VT stimulation in all bimodal neurons (integrative and non-integrative) across four age groups. A Kruskall-Wallis non-parametric ANOVA (KW) followed by post-hoc Kolmogorov-Smirnov tests showed that all groups were significantly different from each other (p<0.05 for all comparisons). These data show that both integrative and non-integrative bimodal responses exhibit developmental changes that shift from primarily suppressive responses in neurons from infants to progressively higher response changes in adolescents.
Fig. 6: (A) For all subthreshold multisensory neurons, although present in each age group, their multisensory responses (box/whisker plots for each age group) do not show significant changes in magnitude across the different developmental stages. KW non-parametric ANOVA. Not significant. Only a single subthreshold neuron was observed during infancy (yellow circle). B. For subthreshold multisensory neurons that exhibited response enhancement, the bar graphs (± SE) show that the average magnitude of multisensory enhancement was not affected by developmental age. (Univariate ANOVA. Not significant). C.For subthreshold multisensory neurons that exhibited response depression, the bar graphs (± SE) show that the average magnitude of multisensory depression also was not affected by developmental age (Univariate ANOVA; Not significant).
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