Anatomical and physiological investigation of auditory input to the superior colliculus of the echolocating megachiropteran bat Rousettus aegyptiacus

The objective of this study was to investigate whether a representation of auditory space in the superior colliculus (SC) of the echolocating megachiropteran bat (Rousettus aegyptiacus) exists. Additionally the subcortical auditory connectivity of the SC was investigated. A total of 207 units were recorded in five awake animals while presenting acoustic stimuli (white noise, clicks, and pure tones) at different positions in space. Six units responded to acoustic stimulation. Three of these located within the superficial layers and one located in the intermediate layers were classified as omnidirectional units. Two units were located within the deep layers. One was classified as a hemifield unit, and the other as a frontal unit. All units responded phasically to acoustic stimulation with a latency of 4–150 ms. None of them could be activated by visual stimuli. We further examined the interaction of paired auditory and visual stimulation in 116 visually responsive units. Responses to visual stimulation were markedly altered by acoustic stimulation in 5 units. The influence of the acoustic stimuli was temporally and spatially restricted, and resulted either in a reduction or an elevation of unit responsiveness. Horseradish peroxidase was injected into the SC of eight animals to investigate the auditory subcortical connectivity of the SC. Retrograde labeling in auditory structures was rare compared with labeling found in nonauditory structures (e.g., retina, substantia nigra, parabigeminal nucleus). In auditory structures retrograde labeling was found mainly in the external nucleus of the inferior colliculus and in the nucleus of the brachium of the inferior colliculus. To a lesser extent it was found in the nucleus sagulum and in the area medial to the lemniscal nuclei. In one case the dorsal nucleus of the lateral lemniscus and the anterolateral periolivary nucleus were labeled. Our results reveal only a sparse auditory input into the SC of the flying fox, R. aegyptiacus. On the basis of single-unit recordings, we did not find an elaborate representation of auditory space as it is described for several other species. The existence of auditory and bimodal neurones, in combination with their response properties, nonetheless indicate that there might be a representation of auditory space in the SC of R. aegyptiacus.     

Time of origin of neurons of the rat inferior colliculus and the relations between cytogenesis and tonotopic order in the auditory pathway

Summary Groups of pregnant rats were injected with two successive daily doses of ³H-thymidine from gestational day 12 and 13 (E12+13) until the day before parturition (E21+22) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labelled) on specific embryonic days was determined quantitatively in six vertical strips of the inferior colliculus. It was established that the neurons of the inferior colliculus are produced between days E14 and the perinatal period in an orderly sequence: the earliest generated cells are situated rostrally, laterally and ventrally in the principal nucleus, the latest generated cells are situated caudally, medially and dorsally in the pericentral nucleus. This cytogenetic gradient suggested that the cells are produced dorsally in the caudal recess of the embryonic aqueduct and are deployed in an outsidein pattern.This study has brought to a conclusion our datings of neuron production in the central auditory pathway of the rat. The results revealed that in those structures in which a cytogenetic gradient could be recognized, the orientation of this gradient and the regional tonotopic order (demonstrated mostly in species other than the rat) tended to be aligned. Moreover, with the exception of the medial trapezoid nucleus and the dorsal nucleus of the lateral lemniscus (which receive contralateral input from the cochlear nuclei), sites with early-produced neurons correlated with units responding preferentially to high frequency tones and vice versa. This suggested that the orderly production of neurons within different components of the auditory system is a factor in their subsequent topographic organization. A comparison of the temporal order of neuron production in different components of the auditory pathway suggested that the establishment of orderly topographic relations between some of the structures (e.g., the medial geniculate body and the primary auditory cortex) takes place before this spatial relationship could be specified as a cochleotopic order.Abbreviations ab cochlear nerve, ascending branch - Ai aqueduct, inferior collicular recess - AI primary auditory cortex - bi brachium of inferior colliculus - c caudal - CE cerebellum - CI central nucleus, inferior colliculus - CNa anteroventral cochlear nucleus - CNd dorsal cochlear nucleus - CNp posteroventral cochlear nucleus - d dorsal - db cochlear nerve, descending branch - DI diencephalon - ds dorsal acoustic stria (stria of Monakow) - DM dorsomedial nucleus, inferior colliculus - EX external nucleus, inferior colliculus - IC inferior colliculus - is intermediate acoustic stria (stria of Held) - l lateral - LD dorsal nucleus of lateral lemniscus - ll lateral lemniscus - LV ventral nucleus of lateral lemniscus - m medial - ME medulla - MG medial geniculate body - MS mesencephalon - PC pericentral nucleus, inferior colliculus - PR principal nucleus, inferior colliculus - py pyramidal cells, dorsal cochlear nucleus - r rostral - SOl lateral superior olivary nucleus - SOm medial superior olivary nucleus - TRl lateral trapezoid nucleus - TRm medial trapezoid nucleus - v ventral - VL lateral ventricle - vs ventral acoustic stria (trapezoid body) - V3 third ventricle - VIIIn cochlear nerve     

An electrophysiological study of neural pathways for corticofugally inhibited neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicus fuscus

This electrophysiological study tests the hypothesis that one possible neural pathway for corticofugally inhibited neurons in the central nucleus of the inferior colliculus (ICc) of the big brown bat, Eptesicus fuscus, is mediated through excitatory projections from the auditory cortex (AC) to the external nucleus of the IC (ICx), which then sends inhibitory inputs to the ICc. This study shows that all neurons in the ICx are broadly tuned to stimulus frequency. Electrical stimulation in the AC typically increases the number of impulses, expands the auditory spatial response areas, and broadens the frequency tuning curves (FTCs) of neurons in the ICx. This corticofugal facilitation is mediated at least in part through NMDA receptors, since application of DL-2-amino-5-phosphonovaleric acid (APV), an antagonist for NMDA, decreases these response properties of neurons in the ICx. Electrical stimulation in the ICx typically decreases the number of impulses, reduces the auditory spatial response areas, and narrows the FTCs of neurons in the ICc. This inhibition is mediated at least in part through GABA_A receptors, since application of bicuculline, an antagonist for GABA, increases these response properties of neurons in the ICc. These data suggest that corticofugal facilitation of the ICx and the inhibition of the ICx to the ICc may be one of the polysynaptic pathways for corticofugal inhibition of neurons in the ICc. Possible functions of this polysynaptic pathway in acoustic orientation and signal processing are discussed. Electronic Publication     

Changes induced in the representation of auditory space in the superior colliculus by rearing ferrets with binocular eyelid suture

There have been conflicting reports concerning the importance of visual experience in the development of auditory localization mechanisms. We have examined the representation of auditory space in the superior colliculus of adult ferrets that were visually deprived by binocular eyelid suture from postnatal days 25–28, prior to natural eye opening, until the time of recording. This procedure attenuated the transmission of light by a factor of at least 20–25 and blurred the image so that, as long as the eyelids were still fused, the responses of visual units in the superficial layers of the superior colliculus were labile and very poorly tuned. After the eyelids were opened, the representation of the visual field in these layers appeared to be normal. Acoustically responsive units were, as usual, almost exclusively restricted to the deeper layers of the superior colliculus. However, unlike normal animals, where responses occurring only at stimulus onset pre-dominate, most of these units exhibited sustained or multi-peaked discharge patterns. The degree of spatial tuning of individual units recorded from the normal and deprived groups of animals was not significantly different in either azimuth or elevation. Normally orientated maps of both sound azimuth and elevation were also found in the visually deprived ferrets. However, abnormalities were present in the topography and precision of these representations and consequently in their alignment with the overlying visual map. In particular, an increase was observed in the proportion of auditory units with spatially ambiguous receptive fields, in which the maximum response occurred at two distinct locations. These results indicate that patterned visual experience is not required for establishing at least a crude map of auditory space in the superior colliculus, but suggest that it may play a role in refining this representation during development.     

Peripheral and central stimulus detection in auditory pathways of monkeys

Monkeys were trained in an auditory signal detection paradigm to detect increments in noise intensity. The task was analogous to the yes-no detection task in that two responses were predicated on the occurrence of the intensity increment, and the behavior was analyzed with respect to sensitivity and response bias. Central processing of peripheral stimulus information was investigated by examining stimulus evoked potentials at inferior colliculus and by conditioning or replacing the peripheral stimulus with central electrical stimulation at the inferior colliculus or auditory cortex. Evoked potentials at the colliculus reflected increment presentation but did not predict responses. Central conditioning stimuli of long duration resulted in sensitivity decrements indicating some interference with information processing at colliculus and cortex as well as producing some change in bias. Discrete, 5 msec conditioning pulses led to pure bias changes under appropriate timing conditions suggesting additive interaction of central and peripheral stimulus information. Receiver operating characteristics were generated with collicular and cortical stimuli serving as the signal indicating that such stimulation affects memory as does peripheral stimulation. Stimulation at parietal and occipital lobes proved ineffective for conditioning or replacing peripheral stimuli. Implications for serial versus parallel processing are discussed.     

Use of pseudorandom noise in studies of auditory evoked potentials

The extent to which sensorineural systems such as the auditory system are nonlinear depends on the type of stimulus that is used, and the part of the system from which recordings are made. An estimate of the first-order Wiener kernel of the evoked response from the inferior colliculus to amplitude-modulated tones and noise was obtained by cross-correlating the response with the same pseudorandom noise as was used to amplitude modulate the sounds that were used as stimuli, in order to characterize the linear portion of the system. The shape of these cross-correlograms resembled the potentials evoked to short bursts of the unmodulated tones and noise. The degree of nonlinearity in the response to amplitude-modulated tones and noise was determined, and information about the type of nonlinearity was obtained using the inverse-repeat feature of the pseudorandom noise. Recordings both from the surface and from deep in the nucleus of the inferior colliculus revealed nonlinearities that were predominantly of an even order, but the magnitude of the nonlinearities depended on what stimulus was used, the stimulus intensity, and from which neural structure the recording was made.     

豚鼠颞叶出血前后听觉中潜伏期反应变化

目的：观察豚鼠颞叶出血前后听觉中潜伏期反应（MLR）变化。方法：通过豚鼠未肝素化自体血回注建立颞叶出血模型，监测其出血前后听觉MLR的主要波形A波的变化。结果：①A波起源于颞部听皮层，正常豚鼠潜伏期大约在12～15ms之间；②颞叶出血后A波潜伏期（17～23ms）明显延长，甚至A波消失。结论：MLR的A波是反映颞叶出血的早期、客观、无创伤的一项重要指标。     

Effects of sleep deprivation on auditory change detection: a N1-Mismatch Negativity study

The effects of sleep deprivation on neural activity underlying stimulus change detection are still debated. The aim of this study was to investigate the effects of sleep deprivation on the relationship between N1 refractoriness and Mismatch Negativity (MMN) as indexes of different stages of change detection. Respectively, N1 represents the sensory feature trace creation with stimulus repetition and MMN represents the memory-based detection of deviance in a new incoming stimulus. Event-related potentials (ERPs) were recorded from 22 healthy participants during a passive auditory oddball task after a night of normal sleep and after a night of total sleep deprivation (TSD). Importantly, stimulus presentation was organized as a train of 10 stimuli, so that N1 refractoriness could be measured as amplitude decrease with stimulus repetition within each train. Results showed that N1 refractoriness and MMN were not affected by TSD suggesting that the change detection process was preserved in our paradigm. However, the overall N1 amplitude increased after TSD, an effect that may be related to an enhancement of cortical excitability.     

Firing of inferior colliculus auditory neurons is phase-locked to the hippocampus theta rhythm during paradoxical sleep and waking

The activity of 52 single auditory units in the central nucleus of the inferior colliculus (IC) was recorded along with cortical and hippocampal (CA1) electrograms and neck muscle electromyograms in behaving, head-restrained guinea pigs during paradoxical sleep (PS) and wakefulness. Sixteen (30%) of the IC auditory units showed positive correlation with the hippocampal theta () rhythm: 8 (15%) were rhythmic with phase-locking (type 1), 8 (15%) showed only phase-locking with no rhythmicity (type 2), while 70% did not show any correlation to hippocampal rhythm (type 3). During wakefulness IC neurons (4 of 13) showed a higher synchrony with hippocampal when sound-stimulated at the unit's characteristic frequency. During PS all IC auditory neurons recorded presented some hippocampal correlation: 40% were rhythmic and phase-locked to the frequency and 60% were nonrhythmic maintaining the phase-locking. Shifts in the angle of phase-locking to the rhythm were observed during PS. It is suggested that the hippocampal rhythm may play the part of an internal clock, adding a temporal dimension to the processing of auditory sensory information.     

氨基甲酰促红细胞生成素对氧糖剥夺条件下神经干细胞保护作用的研究

目的 研究氨基甲酰促红细胞生成素（CEPO）对体外神经干细胞缺氧缺血性损伤的保护作用.方法 从孕14-16 d大鼠获得神经干细胞,经过培养并传代,对所获细胞的自我增殖、自我更新及多分化潜能进行检测.取第3代神经干细胞中添加CEPO,在氧糖剥夺（OGD）条件下培养2 h.通过TUNEL法计数神经干细胞凋亡率和MTT法检测神经干细胞的存活情况.采用酶联免疫吸附试验（ELISA）法检测神经干细胞分泌IFN-γ的情况.应用流式细胞仪检测MHC-Ⅱ在神经干细胞中的表达.结果 在OGD环境中,细胞凋亡率达58.97%,存活率为39.46%,IFN-γ和MHC-Ⅱ类分子的表达分别为78.47 pg/ml和35.68%,加入CEPO后神经干细胞的凋亡率下降至30.15%,存活率升高至75.84%,IFN-γ（15.35 pg/ml）和MHC-Ⅱ类分子的表达均下降（9.77%）.结论 CEPO对神经干细胞缺氧缺血性损伤具有明显的保护作用,而且对炎症分子IFN-γ和免疫分子MHC-Ⅱ类分子的表达具有调节作用.     

Plasticity of tonotopic maps in auditory midbrain following partial cochlear damage in the developing chinchilla

 There is substantial reorganization of the midbrain (inferior colliculus) tonotopic map following neonatally induced partial cochlear lesions in the chinchilla. The most obvious feature of this remapping is a large ”iso-frequency” region in the ventral sector of the central nucleus of inferior colliculus (ICC). Neurons in this region exhibit similar threshold and tuning properties, with a common characteristic frequency which corresponds to the high-frequency audiometric cutoff. This overrepresented frequency range also corresponds to the high-frequency border of the cochlear lesion. Alterations to the tonotopic map corresponding to lower frequencies, in more dorsal regions of ICC, depend on the extent and degree of the cochlear lesion. When there is minimal damage to apical (low-frequency) cochlear areas, the dorsal ICC has relatively normal frequency representations. With more extensive apical cochlear lesions there is a corresponding disruption of ICC tonotopic representation of the low frequencies. We conclude that the tonotopic map within the ICC can become (re)organized postnatally according to the abnormal pattern of neural activity from the auditory periphery. Similar reorganization can be expected to occur in human infants with a partial cochlear hearing loss from birth. Received: 2 February 1998 / Accepted: 3 July 1998     

Visual movement and pattern are important for the development of a map of auditory space in the guinea pig superior colliculus

Previous data have indicated that, if guinea pigs are deprived of all visual information during a crucial period early in development (26–30 days after birth), the map of auditory space in the superior colliculus (SC) is completely disrupted. In the experiments reported here, multi-unit auditory receptive fields were recorded in the SC of two groups of anaesthetised guinea pigs that had been exposed to different forms of visual deprivation. One group was reared in a movement-free environment (strobe-reared) and the other group was reared in a pattern-free environment (their eyes covered with light-diffusing masks). Both groups experienced visual restriction during the crucial period for auditory space map development. In both experimental groups, the multi-unit auditory receptive fields were broad and all spatial tuning parameter values were significantly greater than the equivalent values from a control group of normal animals. In the patterns and motion-deprived groups, a significant correlation existed between the rostro-caudal position of the recording electrode in the SC and the peak response angle of the receptive field, thus showing a degree of topographic organisation of the auditory receptive fields in the SC. However, the topographic order was less precise than that displayed by the control group of animals. These results indicate that, during development, both visual pattern and movement are important for the refinement of the SC auditory space map in the guinea pig.     

Frequency response areas of neurons in the mouse inferior colliculus. I. Threshold and tuning characteristics

Excitatory and inhibitory frequency response areas of 130 neurons of the central nucleus of the mouse inferior colliculus (ICC) were mapped by extracellular single-unit recordings and quantitatively evaluated with regard to thresholds, steepness of slopes of excitatory tuning, characteristic frequencies of excitation (CF_E), inhibition (CF_I), and bandwidths of response areas (sharpness of tuning). Two-tone stimuli were used to determine the shapes of inhibitory response areas. Class I neurons (n=54) had asymmetrical (with regard to the CF_E) excitatory and inhibitory response areas, with inhibition above CF_E having lower thresholds and covering larger areas than inhibition below CF_E. Quantitative relationships between CF_E and CF_I thresholds, and sharpness of tuning showed that the receptive fields of about two-thirds of these neurons had properties similar to auditory nerve fibers. Class II neurons (n=36) had small symmetrical or tilted excitatory areas of rather constant bandwidths and broad inhibitory areas reaching far into and often through the excitatory area, leading to closed excitatory areas in ten neurons. Class III neurons (n=32) had higher excitatory thresholds and the highest proportions of unilateral inhibitory areas compared with neurons of the other classes. Their excitatory area often widened symmetrically with increasing sound level. Their inhibitory areas did not overlap with the excitatory area. Class IV neurons (n=8) had two branches of excitatory areas (two-CFs_E) and six of the neurons had a central inhibitory area in addition to the low- and high-frequency inhibitory areas. In most neurons, the shapes of excitatory response areas predicted the shapes of inhibitory areas. Altogether, 15 neurons from all 4 classes had areas of facilitation in addition to inhibitory areas. Facilitation in six class IV neurons occurred between the two branches of the excitatory area. All 130 neurons had large inhibitory areas, 106 of them on both sides of the excitatory area. That is, sound processing in the ICC shows strong inhibitory components. The close relationships between excitatory and inhibitory CFs found here indicate that inhibitory projections to and interactions within the ICC are tonotopically organized comparable to the excitatory ones. Electronic Publication     

Effects of medial olivocochlear efferent stimulation on the activity of neurons in the auditory midbrain

Medial olivocochlear (MOC) efferents are known to suppress spontaneous activity and sound-evoked responses of primary afferents by their actions on outer hair cells in the cochlea. This study investigated the effects of MOC activation on the responses of single neurons in the central nucleus of the inferior colliculus (CNIC) of anaesthetized guinea pigs. Extracellular responses of CNIC neurons to contralateral tones were recorded with and without MOC stimulation in normal animals and in animals acutely treated with gentamicin to eliminate peripheral effects of MOC activation. In normal animals, input–output functions of CNIC neurons showed a variety of changes. Some effects resembled qualitatively those reported for primary afferents. However, other effects were also observed, including an increase of firing rates at medium- to high-tone levels and in a small number of neurons (10%), an increase in spontaneous activity. In addition, larger threshold shifts and larger reductions of spontaneous firing rates were observed as compared to effects seen in the periphery. In gentamicin-treated animals, activation of MOC efferents did not produce any changes in the input–output functions or spontaneous activity of CNIC neurons. This observation is consistent with the majority of MOC-induced changes in monaural responses in the CNIC being mediated by the actions of MOC terminals in the cochlea and resulting from the interplay between altered afferent input and central circuitry.     

Neural connectivity only accounts for a small part of neural correlation in auditory cortex

In order to allow the relation of functional connectivity patterns (inferred from cross-correlograms) to structural connectivity (the anatomical substrate), we analyzed cross-correlogram peaks for spontaneous and stimulated activity in the auditory cortex. It was assumed that the broad correlograms, usually encountered, represent neural connectivity as well as secondary effects such as intrinsic firing patterns, global synchrony related to the ongoing electroencephalographic activity, and stimulus-related effects. Data were collected from 604 neuron pairs recorded under spontaneous conditions in primary auditory cortex of seven juvenile (30–70 days) and nine adult cats. Three hundred and six pairs (51%) had a peak cross-correlation coefficient significantly different from zero. For 113 neuron pairs out of this subgroup, correlations were calculated also for spike trains recorded during click stimulation. After a combined burst-correction and deconvolution procedure was carried out, the correlation peak strengths were not significantly changed for spontaneous activity, but peak width was narrower for single-electrode pairs than for dual-electrode pairs, suggesting a better synchronization for neighboring neurons. Under click stimulation conditions, overall peak synchronization strength was independent of interelectrode distance, whereas, after correction for secondary and stimulus effects, peak synchronization was significantly lower for dual-electrode pairs. However, the primary peak width for single-electrode pairs under stimulus conditions was no longer different from that of dual-electrode pairs. This implies that both under spontaneous and stimulus conditions secondary effects largely obscure any underlying correlation produced by anatomical connectivity. The secondary effects may be the result of intrinsic as well as network properties in auditory cortex and may functionally be more important than the weak primary effects resulting from anatomical connections. Cross-interval analysis suggests that the correlations in auditory cortex are dynamic and may show random switching between states of stronger and weaker synchronization.     

Stimulus intensity modifies saccadic reaction time and visual response latency in the superior colliculus

Performance in a reaction time task can be strongly influenced by the physical properties of the stimuli used (e.g., position and intensity). The reduction in reaction time observed with higher-intensity visual stimuli has been suggested to arise from reduced processing time along the visual pathway. If this hypothesis is correct, activity should be registered in neurons sooner for higher-intensity stimuli. We evaluated this hypothesis by measuring the onset of neural activity in the intermediate layers of the superior colliculus while monkeys generated saccades to high or low-intensity visual stimuli. When stimulus intensity was high, the response onset latency was significantly reduced compared to low-intensity stimuli. As a result, the minimum time for visually triggered saccades was reduced, accounting for the shorter saccadic reaction times (SRTs) observed following high-intensity stimuli. Our results establish a link between changes in neural activity related to stimulus intensity and changes to SRTs, which supports the hypothesis that shorter SRTs with higher-intensity stimuli are due to reduced processing time.     

The normal scalp topography of the middle latency auditory evoked potential Pa component following monaural click stimulation

Summary The scalp topography of the middle latency auditory evoked potential (MLAEP) Pa component following left and right ear click stimulation was investigated in 15 normal hearing and neurologically intact right-handed subjects. An unbiased reference was employed. The Pa component showed a broad voltage field that was recorded maximally at the Cz and Fz leads regardless of which ear was stimulated. A broad negative voltage field that occurred coincident in time with the Pa component was recorded posterior to the T3, P3, Pz, P4 and T4 electrode leads. This negative voltage field peaked in amplitude at the T5, O1, Oz, O2, and T6 electrode leads. An unexpected finding was that the peak latency of the Pa component occurred significantly earlier following stimulation of the right ear.     

Development of inferior colliculus response properties in C57BL/6J mouse pups

Summary Response properties of inferior colliculus (IC) neurons were studied in tranquilized C57BL/6J mice during a period of rapid auditory system development between 12 and 17 days of age. In IC units of the youngest mice, spontaneous activity was absent, a disproportionate number of onset responses was observed, and many units were not securely driven by sound. Frequency response ranges were restricted to relatively low frequencies, sharpness of tuning was poor, and thresholds at best frequencies (BFs) were quite high. Dynamic intensity ranges were restricted, but nonmonotonic functions were observed. By 15–17 days of age, spontaneous activity was appreciable, incidences of response patterns were near adult proportions, and most units in the ventrolateral nucleus were securely driven by tones. Response ranges had expanded markedly to include high frequencies, sharpness of tuning increased, and thresholds had decreased. Dynamic intensity ranges and intensity functions were similar to those observed in adult mice.     

Neural responses to free-field auditory stimulation in the superior colliculus of the wallaby (Macropus eugenii)

Auditory responses to free-field broad band stimulation from different directions were recorded from clusters of neurones in the superior colliculus (SC) of the anaesthetized tammar wallaby. The auditory responses were found approximately 2 mm beneath the first recording of visually evoked responses in the superficial layers, the vast majority being solely auditory in nature; only one recording responded to both auditory and visual stimulation. Responses to suprathreshold intensities displayed sharp spatial tuning to sound in the contralateral hemifield. Those from the rostral pole of the SC disclosed a preference for auditory stimuli in the azimuthal anterior field, whereas those in the caudal SC preferentially responded to sounds in the posterior field. A continuum of directionally tuned responses was seen along the rostrocaudal axis of the SC so that the entire azimuthal contralateral auditory hemifield was represented in the SC. Furthermore, tight spatial alignment was evident between the best position of the visual responses in the superficial layers in azimuth and the peak angle of the auditory response in the deeper layers.     

Integration of visual and auditory information in bimodal neurones in the guinea-pig superior colliculus

Summary We have investigated the responses of neurones in the guinea-pig superior colliculus to combinations of visual and auditory stimuli. When these stimuli were presented separately, some of these neurones responded only to one modality, others to both and a few neurones reliably to neither. To bimodal stimulation, many of these neurones exhibited some form of cross-modality interaction, the degree and nature of which depended on the relative timing and location of the two stimuli. Facilitatory and inhibitory interactions were observed and, occasionally, both effects were found in the same neurone at different inter-stimulus intervals. Neurones whose responses to visual stimuli were enhanced by an auditory stimulus were found in the superficial layers. Although visual-enhanced and visual-depressed auditory neurones were found throughout the deep layers, the majority of them were recorded in the stratum griseum profundum. Neurones that responded to both visual and auditory stimuli presented separately and gave enhanced or depressed responses to bimodal stimulation were found throughout the deep layers, but were concentrated in the stratum griseum intermediale and extended into the stratum opticum.