The planum temporale as a computational hub
Trends in Neurosciences
What does the human planum temporale (PT) do? This large region, which occupies the superior temporal plane posterior to Heschl's gyrus, is generally agreed to represent auditory association cortex. However, disagreement exists regarding its anatomy  and structure-function relationships    . In the left hemisphere, most definitions of Wernicke's area include part of PT  and, indeed, the human PT has traditionally been viewed as a language processor . However, functional
... indicates that the PT processes diverse types of sound ( Fig. 1, Table 1 ). This article develops a functional model to explain this. The PT is concerned with analysis of sounds that are spectrally and temporally complex, comprising several component frequencies that change over time (Fig. 2) . Such sounds are common in nature. The brain is continuously required to analyse these incoming spectrotemporal patterns and to compare them with those previously experienced, during the process known as auditory scene analysis  . Such analysis allows the identification and assignment of position to a mixture of acoustic sources (sound objects) heard simultaneously. This demands both segregation of the spectrotemporal pattern associated with each sound object and separation of each object from the spectrotemporal effects of its location. We argue that the PT solves this daunting computational problem. Although mechanisms for the accurate representation of incoming acoustic spectrotemporal signals exist in the ascending auditory pathways and primary auditory cortex (PAC)    , it would be surprising if a priori this system were sufficient for auditory scene analysis. Even the discrimination of a single sound object from the effect of spatial position (Fig. 3) requires learned information about how the external ears filter sound signals arising in different locations, in addition to accurate representations of the sound waveform at the eardrums. This demanding computation might be achieved serially in the PT after initial processing in PAC, using the modular architecture in the PT  and inputs from other cortical areas     . Such computation would transform incoming auditory patterns into information about acoustic objects and position that could be used in other cortical areas. In this model, the PT thus represents a computational 'hub' that directs further processing in other cortical regions, consistent with studies of the The planum temporale as a computational hub It is increasingly recognized that the human planum temporale is not a dedicated language processor, but is in fact engaged in the analysis of many types of complex sound. We propose a model of the human planum temporale as a computational engine for the segregation and matching of spectrotemporal patterns. The model is based on segregating the components of the acoustic world and matching these components with learned spectrotemporal representations. Spectrotemporal information derived from such a 'computational hub' would be gated to higher-order cortical areas for further processing, leading to object recognition and the perception of auditory space. TRENDS in Neurosciences (a) (b) STG, STS, MTG STG PTO / IPL PTO / IPL L R Speech Environmental sounds Cross-modal processing Auditory space Music Simple sound patterns + Voices Fig. 1. The planum temporale (PT) as an anatomical and functional hub. (a) Tilted axial section through the superior temporal plane of the human brain. The PT lies posterior to Heschl's gyrus (HG), the site of the primary auditory cortex, and is contiguous posteriorly with the parieto-temporal operculum (PTO). Ninety-five percent probability maps for the boundaries of left and right PT in humans (derived from Ref. ) are outlined in red. (b) Insets centred on left and right PT, showing functional activation peaks within PT associated with different types of complex sound processing (see Table 1 ). Symbols are explained underneath. The functional relationships between the PT and higher cortical areas that are coactivated in processing simple sound patterns (green), music (yellow), speech (red) and auditory space (blue) are indicated schematically. Arrows indicate postulated flow of information from the PT to these higher areas; in many cases, however, exchange of information is likely to be reciprocal. We propose a generic computational mechanism within the PT for the analysis of spectrotemporal pattern. Computation uses information about sound objects derived from higher cortical areas linked to the PT, and the output of the PT is used to update stored information in these same areas. Abbreviations: IPL, inferior parietal lobe; MTG, middle temporal gyrus; PTO, parieto-temporal operculum; STG, lateral superior temporal gyrus; STS, superior temporal sulcus.