The dual-stream processing model in audition4,5
has been a useful construct in hearing research, perceptual physiology and, in particular, psycholinguistics, where it has spawned several further models73,74
that have tried to accommodate specific results from this field. The role of a ventral stream in hierarchical processing of objects, as in the visual system, is now widely accepted. Specifically for speech, anterior regions of the superior temporal cortex respond to native speech sounds and intelligible speech, and these sounds are mapped along phonological parameter domains. By contrast, early posterior regions in and around the planum temporale are involved in the processing of many different types of complex sound. Later posterior regions participate in the processing of auditory space and motion but seem to integrate input from several other modalities as well.
Although evidence is strong for the role of the dorsal pathway (including pST) in space processing, the dorsal pathway needs to accommodate speech and language functions as well. Spatial transformations may be one example of fast adaptations used by ‘internal models’ or ‘emulators’, as first developed in motor control theory. Within these models, ‘forward models’ (predictors) can be used to predict the consequences of actions, whereas ‘inverse models’ (controllers) determine the motor commands required to produce a desired outcome88
. More recently, forward models have been used to describe the predictive nature of perception and imagery89
. The IPL could provide an ideal interface, where feed-forward signals from motor preparatory networks in the inferior frontal cortex and premotor cortex (PMC) can be matched with feedback signals from sensory areas72
In speech perception and production, projections from articulatory networks in Broca's area and PMC to the IPL and pST interact with signals from auditory cortex (). The feed-forward projection from Brodmann area 44 (and ventral PMC) may provide an efference copy in the classical sense of von Holst and Mittelstaedt90
, informing the sensory system of motor articulations that are about to happen. This occurs in anticipation of a motor signal if the behavior is enacted, or as imagery if it is not. The activity arriving in the IPL and pST from frontal areas anticipates the sensory consequences of action. The feedback signal coming to the IPL from pST, conversely, could be considered an “afference copy”91
with relatively short latencies and high temporal precision92
—a sparse but fast primal sketch of ongoing sensory events93
that are compared with the predictive motor signal in the IPL at every instance.
‘Internal model’ structures in the brain are generally thought to enable smooth sequential motor behaviors, from visuospatial reaching to articulation of speech. The goal of these models is to minimize the resulting error signal through adaptive mechanisms. At the same time, these motor behaviors also support aspects of perception, such as stabilization of the retinal image and disambiguation of phonological information, thus switching between forward and inverse modes. As Indefrey and Levelt94
point out, spoken language “constantly operates a dual system, perceiving and producing utterances. These systems not only alternate, but in many cases they partially or wholly operate in concert.” What is more, both spatial processing and real-time speech processing make use of the same internal model structures.
In summary, our new model of the auditory cortical pathways builds on the previous model of dual processing pathways for object identification and spatial analysis5,6
, but integrates the spatial (dorsal) pathway with findings from speech and music processing as well. The model is based on neuroanatomical data from nonhuman primates, operating under the assumption that mechanisms of speech and language in humans have built on structures available in other primates. Finally, our new model extends beyond speed processing74
and applies in a very general sense to both vision and audition, in its relationship with previous models of perception and action26,27