Spatial Sensitivity in the Dorsal Zone (Area DZ) of Cat Auditory Cortex

Abstract

We compared the spatial sensitivity of neural responses in three areas of cat auditory cortex: primary auditory cortex (A1), the posterior auditory field (PAF), and the dorsal zone (DZ). Stimuli were 80-ms pure tones or broadband noise bursts varying in free-field azimuth (in the horizontal plane) or elevation (in the vertical median plane), presented at levels 20-40 dB above units' thresholds. We recorded extracellular spike activity simultaneously from 16 to 32 sites in one or two areas of alpha-chloralose-anesthetized cats. We examined the dependence of spike counts and response latencies on stimulus location as well as the information transmission by neural spike patterns. Compared with units in A1, DZ units exhibited more complex frequency tuning, longer-latency responses, increased prevalence and degree of nonmonotonic rate-level functions, and weaker responses to noise than to tonal stimulation. DZ responses also showed sharper tuning for stimulus azimuth, stronger azimuthal modulation of first-spike latency, and enhanced spatial information transmission by spike patterns, compared with A1. Each of these findings was similar to differences observed between PAF and A1. Compared with PAF, DZ responses were of shorter overall latency, and more DZ units preferred stimulation from ipsilateral azimuths, but the majority of analyses suggest strong similarity between PAF and DZ responses. These results suggest that DZ and A1 are physiologically distinct cortical fields and that fields like PAF and DZ might constitute a "belt" region of auditory cortex exhibiting enhanced spatial sensitivity and temporal coding of stimulus features.

Specialization for Sound Localization in Fields A1, DZ, and PAF of Cat Auditory Cortex

“…The across-location ranges of first-spike latencies were given by the differences between the longest and shortest mean first-spike latencies across all stimulus locations. Our presentation of latencies is consistent with that in our previous reports (Stecker et al, 2003;2005b) in that latencies of individual units are given by geometric means, whereas the central tendencies of distributions across units is given by medians.…”

“…That is, all units recorded at a DZ array placement were counted as DZ units even if, for instance, one or more of them showed responses more typical of A1 units. As in our previous acute experiments (Stecker et al, 2003;2005b), the recording arrays in field A1 were oriented roughly parallel to cortical anatomical columns, whereas those arrays that were placed in DZ and PAF on the banks of sulci tended to cross cortical columns. Two cats received array placements in both right and left hemispheres, whereas arrays were restricted to the right hemisphere in the other three cats; in the illustrations, stimulus locations are shown as contra-or ipsilateral relative to the side of the recording site.…”

“…Unilateral inactivation of PAF or AES, or combined inactivation of A1 and DZ, reduced localization accuracy to around chance levels (Malhotra et al, 2004), whereas selective inactivation of A1 or DZ produced more limited deficits, which differed between fields (Malhotra et al, 2008). Similarly, physiological recordings in α-chloralose-anesthetized cats revealed differences in spatial sensitivity among neurons in fields A1, DZ, and PAF (Stecker et al, 2003;2005b). In those studies, A1 neurons demonstrated relatively broad spatial sensitivity, with sensitivity broadening even further as sound levels increased.…”

“…The placements of arrays in particular cortical areas were confirmed by intraoperative unit recording through the arrays under conditions of isoflurane anesthesia. In particular, DZ units were distinguished from A1 units by the DZ units' characteristic broader frequency tuning biased toward high frequencies and by their longer latencies: A1 units typically responded briskly with first-spike latencies G20 ms, whereas DZ latencies tended to be longer and more variable (Middlebrooks and Zook, 1983;Stecker et al, 2005b). We attempted to place A1 and DZ arrays near the estimated centers of those fields.…”

Cortico‐cortical connectivity within ferret auditory cortex

“…It is therefore not immediately clear which fields should be considered as homologous across these species based on the direction of the tonotopic gradients. The responses of neurons in the PAF are often nonmonotonic with respect to sound intensity and have longer latencies and more sustained firing patterns (Stecker et al, 2005), whereas the VPAF response properties remain relatively undocumented. These properties of PAF neurons resemble those of neurons in both the PPF and PSF in the ferret (Bizley et al, 2005).…”