K. E. Binns

Guinea pigs, reared from birth in an environment of omnidirectional white noise, fail to develop a map of auditory space in the deeper layers of the superior colliculus. Collicular responses from such noise-reared animals reveal large auditory spatial receptive fields. The representation of auditory space in the colliculus shows no topographic order. Exposing developing animals to the noise environment only for restricted time periods showed that animals reared normally up to 26 days after birth (DAB) and then placed in the noise chamber could not construct spatial maps, whereas animals reared normally to 30 DAB and then placed in the noise chamber until the terminal mapping experiment could construct topographically organized spatial maps with local receptive fields. Limiting the noise exposure to the period between 26 and 30 DAB was sufficient to prevent spatial map formation. The failure to form a map of auditory space did not reflect environmental damage to the cochlea or the functional organization of the primary auditory pathway. The response thresholds of cochlear microphonics and of auditory responses in both the inferior and superior colliculus were normal in noise-reared animals. Similarly normal were the tonotopic organization and frequency tuning characteristics of inferior collicular neurons. The rearing environment thus appears to exert a selective effect upon the maturation of the superior collicular map of auditory space. We attribute this effect to the masking, by the omnidirectional broad-band noise, of discrete localized auditory stimuli. Cues deriving from these latter stimuli would appear to be necessary for the elaboration of the map of auditory space. This auditory experience operates during a 4 day crucial developmental period from 26 to 30 DAB. This is the same developmental time window as that during which visual experience is required for the construction of the map.

1. The superficial grey layer of the superior colliculus (SGS) contains a high proportion of GABAergic inhibitory neurones. We have investigated the role of GABA receptors in synaptic transmission of aspects of visual activity in the SGS that may be driven by inhibitory mechanisms, such as surround inhibition and response habituation. 2. Multi-barrel glass iontophoretic pipettes were used to record single neuronal activity in the SGS of urethane-anaesthetized rats. Visual stimulation was provided by the display of moving bars and stationary spots of light on a monitor placed in the receptive field. 3. Both ejection of GABA and the GABAB agonist baclofen reduced responses to moving bars (interstimulus intervals > or = 8 s). The effects of GABA were reversed by the GABAA antagonist bicuculline, and the effects of baclofen were antagonized by the GABAB antagonist CGP 35,348. 4. Surround inhibition was estimated by plotting the response to flashed spots of increasing diameter. In controls, expanding the spot diameter beyond the excitatory receptive field caused a decrease in the response. This inhibitory surround was reversibly reduced by bicuculline, but CGP 35,348 had no effect. 5. Response habituation is the progressive reduction in the visual response during repetitive stimulus presentation. In controls, the visual response was reduced to 44 +/- 3% of its initial level when a stimulus (moving bar) was presented 5 times with an interstimulus interval of 0.5 s. During CGP 35,348 ejection, response habituation was reversibly reduced. Bicuculline had no effect on response habituation. 6. The effects of bicuculline on surround inhibition in the superior colliculus are consistent with similar studies in the lateral geniculate nucleus which indicate that GABAA receptors mediate this effect. The function of GABAB receptors in the visual system is less well researched. The reduction of response habituation with CGP 35,348 demonstrates that, at least in the SGS, GABAB receptors have an important role in visual transmission which is distinct from that of GABAA receptors.