Functional subdivisions in the auditory cortex of the guinea pig

Abstract

The auditory fields in the cortex of the guinea pig were investigated with microelectrode mapping techniques. Pure tones of varying frequencies and amplitudes were used as acoustic stimuli. Mainly, multiunit activity was recorded.A large tonotopic area is found in the anterior half of the auditory cortex. This area is named the anterior field (field A). Frequency tuning curves of multiunits in field A are generally narrow. Responses to tone stimuli are strong, and latencies are short. Low best frequencies are represented rostrally, high best frequencies caudally. The tonotopy is continuous and quite regular. Field A is narrow dorsally and becomes gradually broader ventrally. Correspondingly, the isofrequency lines slightly diverge from dorsal to ventral.Caudal to the first field, there is a second, smaller tonotopic area. It lies in the dorsal half of the posterior auditory cortex and is therefore named the dorsocaudal field (field DC). The frequency specificity of the cell clusters in this area is as strong as in field A, but the tonotopy is discontinuous: In the dorsal half of field DC, high best frequencies (16-32 kHz) are represented rostrally; the low frequencies (0.5-2.8 kHz) are represented immediately caudal to the high frequencies, while the intermediate frequencies are missing. Ventrally in field DC, the frequency representation is more complete. Except for this discontinuous map, we did not notice any differences between fields A and DC. A third tonotopic field was found rostral to field A. This field extends over a surface of less than 1 mm2 and was named the small field (field S). It contains a complete representation of the frequency range; high best frequencies are located rostrally, low frequencies caudally. The response latencies are slightly longer in field S than in fields A or DC, and the tuning curves are broader.A broad strip of nontonotopic cortex (auditory belt) surrounds fields A and DC caudally. We subdivided this area into the dorsocaudal and the ventrocaudal belt region. In both areas, tuning curves are often broad, and response latencies are longer than in the tonotopic cortex. In the dorsocaudal belt, most multiunits react with a phasic on-response to pure tones; in the ventrocaudal belt, tonic responses occur more frequently. Another nontonotopic region is located in the anterior auditory cortex, rostral to the tonotopic fields, and was therefore named the rostral belt. Tuning curves in this area are broad, latencies are short, and reponse thresholds are often high.In the discussion, the guinea pig is compared with other mammalian species. Species-specific features in the organization of the tonotopic cortex of the guinea pig are revealed.

Auditory Cortical Images of Tones and Noise Bands

“…A previous study in the guinea pig auditory cortex showed that the low-frequency slopes of tuning DISCUSSION curves tend to be shallower than the high-frequency slopes (Redies et al 1989). Our results are generally consistent with the results obtained from optical The results of our studies demonstrate that (1) pure tone stimuli elicit a restricted focus of activity along imaging of intrinsic signals (Bakin et al 1996).…”

“…Again, Previous studies have examined the frequency responses of auditory cortical neurons in the guinea the network median errors were about half the chance levels. The accuracy of center-frequency identification pig (Hellweg et al 1977;Redies et al 1989). The current study confirms the previously reported distribuand the variance of that measure showed no systematic dependence on stimulus bandwidth.…”

“…The cortical image broadarray. In each of three animals in which two parallel probe placements were made, the progression of ened further caudally than rostrally leading to a slight We computed the cortical scale factor between cortical place and stimulus frequency by calculating the slope of the plot of centroid location versus tone frequency ( m/oct) across 11 probe placements in ten factor is close to the value of ϳ 375 m/oct that can be estimated from the BF data illustrated by Redies and colleagues (Redies et al 1989, Fig. 1).…”

“…In the guinea pig area A1, neurons sensitive to high frequencies are situated lated half-maximal spike count for that stimulus sound level. In cases in which the spike-count-versus-fredorsocaudally, and low-frequency neurons are situated ventrorostrally (Hellweg et al 1977;Redies et al 1989).…”

Functional Subregions in Primary Auditory Cortex Defined by Thalamocortical Terminal Arbors: An Electrophysiological and Anterograde Labeling Study

“…Other functional maps in AI include binaural interaction bands (Imig and Adrian, 1977;Middlebrooks et al, 1980), threshold/intensity (Tunturi, 1950;Schreiner et al, 1992), and sharpness of tuning (Schreiner and Mendelson, 1990). Interestingly, these maps are patchy or discontinuous in their distribution relative to the frequency map Brugge and Imig, 1978;Middlebrooks et al, 1980;Reale and Imig, 1980;Redies et al, 1989;Schreiner, 1991Schreiner, , 1992Schreiner et al, 1992Schreiner et al, , 2000Mendelson et al, 1993Mendelson et al, , 1997Fitzpatrick et al, 1998).…”