DISCUSSION
These results show that multisensory neurons are present in ferret PPr cortex at developmental stages that precede sexual maturity (<~P120). Designated as the “infant” period, multisensory neurons at that time, however, were the minority of neurons observed and only one showed MSI (which was the singular subthreshold neuron identified in this age group). At this early developmental stage, these data from infant, pre-pubertal ferrets closely resemble findings in cat cortex (Wallace et al. , 2006), as well as superior colliculus (Wallace & Stein, 1997), where early multisensory neurons respond independently to stimulation from more than one sensory modality but fail to integrate responses when those cues are presented together. Thus, despite the presence of multisensory convergence, the capacity to integrate multisensory responses apparently is not yet developed in such young cats or ferrets.
For the series of older ferrets (e.g., >P120), the presence of multisensory neurons progressively increased, as was also seen in developing cats following the epoch in which the first multisensory neurons appeared (Wallace et al. , 2006). In the present study, this period is referred to as “adolescence” because, like post-pubertal humans, neural development continues beyond the age of sexual maturity. It is well known that neural maturity in humans is delayed for approximately a decade after sexual onset, especially for higher-order regions, such as inferior parietal and frontal cortices (Spear, 2000a; Spear, 2000b). To our knowledge such conditions have not yet been identified in ferrets (or cats) and hence, there is ambiguity in what is actually defined as being neurally adult in these animals. Indeed, the study of multisensory development in cats did not define neural adulthood, except that the cats designated as adults were older than P180, which is the age of sexual maturity for cats. As a consequence, all the developmental age groups studied in cats were pre-pubertal (Wallace et al. , 2006). In contrast, the ferret age groups in which MSI appeared and developed in bimodal neurons were all post-pubertal adolescents. These conflicting results may be accounted for by the species and methodological differences between these studies, but it might also be possible that PPr neurons mature more slowly than those of the AES perhaps as a reflection of cortical hierarchical differences.
Further evidence that multisensory changes occur in PPr during the ferret adolescent stages comes from the observation that multisensory integration increases in magnitude after the age of sexual maturity at ~P120. Figure 2B summarizes this feature for bimodal neurons that exhibit MSI, and significant increases in multisensory enhancement were demonstrated for stages of adolescence at ages P160-200 and P200-240. Similar developmental increases in multisensory enhancement have also been reported in another study of PPr neurons in early alcohol-treated and control ferrets (Keum et al. , 2023). However, these data do not mirror the MSI levels reported for cats, where the magnitude of multisensory response change observed during development was not different from that seen in adults (Wallace et al. , 2006). It seems possible that these differences in results may be accounted for by the species, regional and methodological differences of these studies.
The present study also examined the levels of multisensory response change that were not significantly different from the most effective unisensory stimulus. It must be kept in mind that these non-integrative responses were evoked in bimodal neurons that were individually activated by visual and by somatosensory stimulation. As a consequence, both stimuli were effective when presented alone and, by definition, were presented within their excitatory receptive fields. Thus, the observed non-integrative effects could not have been induced by inhibitory effects of one stimulus being presented outside of its receptive field. Numerous reports have documented and examined non-integrated multisensory responses in bimodal neurons in a variety of neural areas (Perrault Jr et al. , 2005; Meredith et al. , 2021; Merrikhi et al. , 2022a; Merrikhi et al. , 2022b). The present study shows that such non-integrative multisensory effects were strongly influenced by development because, as depicted in Figure 5D, significant shifts were seen from primarily suppressive levels in infants to progressively higher levels of multisensory response change across the periods of adolescence. Furthermore, similar developmental shifts in non-integrative responses were observed a separate study of PPr neurons (Keum et al. , 2023). Ultimately, the present results show that both non-integrative and integrative response modes of bimodal neurons change during post-pubertal development.
We also examined the subthreshold form of multisensory neuron across the different age groups. Although small in occurrence (as also observed in other areas – (Meredith et al. , 2021; Merrikhi et al. , 2022a; Merrikhi et al. , 2022b), their proportions showed an increasing trend during development but no developmental pattern for changes in MSI levels were demonstrated. A similarly low incidence of multisensory response depression revealed the presence of the effect during development, but consistent changes across development could not be assessed.
In summary, the present study in ferret cortical area PPr identified patterns of appearance of multisensory neurons during development that were similar to those reported for cat cortex (Wallace et al. , 2006). However, unlike the cat AES regions, the magnitude of MSI in ferret PPr significantly increased across the post-pubertal developmental stages. Furthermore, bimodal neurons whose multisensory responses were not integrated showed a consistent shift from suppression in infants to progressively higher levels of multisensory response chage at the adolescent stages. These data, combined with the small presence of subthreshold multisensory neurons give insight into how the population of PPr neurons changes their responses to multisensory cues across pre- and post-pubertal development. How these changes, particularly during adolescence, are parlayed into the behavioral and perceptual tasks of the region are clearly a topic for future investigation.
ACKNOWLEDGMENTS:
Grant sponsor: National Institutes of Health; Grant number: NS064675 (MAM); AA13023 (AEM).