Visual Wulst Research Articles

Innate responses to numerousness reveal neural activation in different brain regions in newly-hatched visually naïve chicks

Whether non-symbolic encoding of quantity is predisposed at birth with dedicated hard-wired neural circuits is debated. Here we presented newly-hatched visually naive chicks with stimuli (flashing dots) of either identical or different numerousness (with a ratio 1:3) with their continuous physical appearance (size, contour length, density, convex hull) randomly changing. Chicks spontaneously tell apart the stimuli on the basis of the number of elements. Upon presentation of either fixed or changing numerousness chicks showed different expression of early gene c-fos in the visual Wulst, the hippocampal formation, the intermediate medial mesopallium, and the caudal part of the nidopallium caudolaterale. The results support the hypothesis that the ability to discriminate quantities does not require any specific instructive experience and involves a neural network with several populations of number-selective neurons. Evidence for innateness of non-symbolic numerical cognition have implications for both neurobiology and philosophy of mathematics.

Digest: How environmental light conditions shape the evolution of visual systems in birds.

How do varying environmental light conditions influence the evolution of avian visual systems? Fröhlich et al. (2024) demonstrate that nocturnal birds evolved broader corneas and slightly longer axial lengths than their diurnal counterparts, increasing light capture efficiency. Nocturnal species also tended to maintain or reduce the size of brain regions responsible for vision, i.e., the optic tectum and the visual wulst. These results highlight adaptive trends in nocturnal species, where evolutionary improvement in low-light performance of eyes may be accompanied by compromised brain function.

GPS tracking technology and re-visiting the relationship between the avian visual Wulst and homing pigeon navigation

Within their familiar areas homing pigeons rely on familiar visual landscape features and landmarks for homing. However, the neural basis of visual landmark-based navigation has been so far investigated mainly in relation to the role of the hippocampal formation. The avian visual Wulst is the telencephalic projection field of the thalamofugal pathway that has been suggested to be involved in processing lateral visual inputs that originate from the far visual field. The Wulst is therefore a good candidate for a neural structure participating in the visual control of familiar visual landmark-based navigation. We repeatedly released and tracked Wulst-lesioned and control homing pigeons from three sites about 10–15 km from the loft. Wulst lesions did not impair the ability of the pigeons to orient homeward during the first release from each of the three sites nor to localise the loft within the home area. In addition, Wulst-lesioned pigeons displayed unimpaired route fidelity acquisition to a repeated homing path compared to the intact birds. However, compared to control birds, Wulst-lesioned pigeons displayed persistent oscillatory flight patterns across releases, diminished attention to linear (leading lines) landscape features, such as roads and wood edges, and less direct flight paths within the home area. Differences and similarities between the effects of Wulst and hippocampal lesions suggest that although the visual Wulst does not seem to play a direct role in the memory representation of a landscape-landmark map, it does seem to participate in influencing the perceptual construction of such a map.

Long-term, high-resolution in vivo calcium imaging in pigeons

Invivo 2-photon calcium imaging has led to fundamental advances in our understanding of sensory circuits in mammalian species. In contrast, few studies have exploited this methodology in birds, with investigators primarily relying on histological and electrophysiological techniques. Here, we report the development of invivo 2-photon calcium imaging in awake pigeons. We show that the genetically encoded calcium indicator GCaMP6s, delivered by the adeno-associated virus rAAV2/7, allows high-quality, stable, and long-term imaging of neuronal populations at single-cell and single-dendrite resolution in the pigeon forebrain. We demonstrate the utility of our setup by investigating the processing of colors in the visual Wulst, the avian homolog of the visual cortex. We report that neurons in the Wulst are color selective and display diverse response profiles to light of different wavelengths. This technology provides a powerful tool to decipher the operating principles that underlie sensory encoding in birds.

Spatial coding in the hippocampus and hyperpallium of flying owls

The elucidation of spatial coding in the hippocampus requires exploring diverse animal species. While robust place-cells are found in the mammalian hippocampus, much less is known about spatial coding in the hippocampus of birds. Here we used a wireless-electrophysiology system to record single neurons in the hippocampus and other two dorsal pallial structures from freely flying barn owls (Tyto alba), a central-place nocturnal predator species with excellent navigational abilities. The owl's 3D position was monitored while it flew between perches. We found place cells-neurons that fired when the owl flew through a spatially restricted region in at least one direction-as well as neurons that encoded the direction of flight, and neurons that represented the owl's perching position between flights. Many neurons encoded combinations of position, direction, and perching. Spatial coding was maintained stable and invariant to lighting conditions. Place cells were observed in owls performing two different types of flying tasks, highlighting the generality of the result. Place coding was found in the anterior hippocampus and in the posterior part of the hyperpallium apicale, and to a lesser extent in the visual Wulst. The finding of place-cells in flying owls suggests commonalities in spatial coding across mammals and birds.

Light-incubation effects on lateralisation of single unit responses in the visual Wulst of domestic chicks

Since the ground-breaking discovery that in-egg light exposure triggers the emergence of visual lateralisation, domestic chicks became a crucial model for research on the interaction of environmental and genetic influences for brain development. In domestic chick embryos, light exposure induces neuroanatomical asymmetries in the strength of visual projections from the thalamus to the visual Wulst. Consequently, the right visual Wulst receives more bilateral information from the two eyes than the left one. How this impacts visual Wulst’s physiology is still unknown. This paper investigates the visual response properties of neurons in the left and right Wulst of dark- and light-incubated chicks, studying the effect of light incubation on bilaterally responsive cells that integrate information from both eyes. We recorded from a large number of visually responsive units, providing the first direct evidence of lateralisation in the neural response properties of units of the visual Wulst. While we confirm that some forms of lateralisation are induced by embryonic light exposure, we found also many cases of light-independent asymmetries. Moreover, we found a strong effect of in-egg light exposure on the general development of the functional properties of units in the two hemispheres. This indicates that the effect of embryonic stimulation goes beyond its contribution to the emergence of some forms of lateralisation, with influences on the maturation of visual units in both hemispheres.

Connectivity between nidopallium caudolateral and visual pathways in color perception of zebra finches

Researchers demonstrated an elegant ability for red discrimination in zebra finches. It is interested to understand whether red activates exhibit much stronger response than other colors in neural network levels. To reveal the question, local field potentials (LFPs) was recorded and analyzed in two visual pathways, the thalamofugal and the tectofugal pathways, of zebra finches. Human studies demonstrate visual associated telencephalons communicate with higher order brain areas such as prefrontal cortex. The present study determined whether a comparable transmission occurs in zebra finches. Telencephalic regions of the thalamofugal (the visual Wulst) and the tectofugal pathway (the entopallium) with their higher order telencephalon, nidopallium caudolateral (NCL) were simultaneously recorded. LFPs of relay nuclei (the nucleus rotundus, ROT) of tectofugal pathway were also acquired. We demonstrated that LFP powers in the tectofugal pathway were higher than those in the thalamofugal pathway when illuminating blue lights. In addition, the LFP synchronization was stronger between the entopallium and NCL. LFPs also revealed a higher Granger causality from the direction of entopallium to NCL and from ROT to entopallium. These results suggest that zebra finches’ tectofugal pathway predominately processing color information from ROT to NCL, relayed by entopallium, and blue could trigger the strongest response.

Self-motion trajectories can facilitate orientation-based figure-ground segregation

Segregation of objects from the background is a basic and essential property of the visual system. We studied the neural detection of objects defined by orientation difference from background in barn owls (Tyto alba). We presented wide-field displays of densely packed stripes with a dominant orientation. Visual objects were created by orienting a circular patch differently from the background. In head-fixed conditions, neurons in both tecto- and thalamofugal visual pathways (optic tectum and visual Wulst) were weakly responsive to these objects in their receptive fields. However, notably, in freely viewing conditions, barn owls occasionally perform peculiar side-to-side head motions (peering) when scanning the environment. In the second part of the study we thus recorded the neural response from head-fixed owls while the visual displays replicated the peering conditions; i.e., the displays (objects and backgrounds) were shifted along trajectories that induced a retinal motion identical to sampled peering motions during viewing of a static object. These conditions induced dramatic neural responses to the objects, in the very same neurons that where unresponsive to the objects in static displays. By reverting to circular motions of the display, we show that the pattern of the neural response is mostly shaped by the orientation of the background relative to motion and not the orientation of the object. Thus our findings provide evidence that peering and/or other self-motions can facilitate orientation-based figure-ground segregation through interaction with inhibition from the surround.NEW & NOTEWORTHY Animals frequently move their sensory organs and thereby create motion cues that can enhance object segregation from background. We address a special example of such active sensing, in barn owls. When scanning the environment, barn owls occasionally perform small-amplitude side-to-side head movements called peering. We show that the visual outcome of such peering movements elicit neural detection of objects that are rotated from the dominant orientation of the background scene and which are otherwise mostly undetected. These results suggest a novel role for self-motions in sensing objects that break the regular orientation of elements in the scene.

Characteristic expressions of the natriuretic peptide family in the telencephalon of juvenile chick

Elucidation of the genes regulating the critical (sensitive) period of imprinting behavior may shed light on the mechanism underlying neural plasticity in early childhood learning. We focused on the family of natriuretic peptides (NPs) as candidates involved in the regulation of the critical period. In avians, several structurally related molecules comprised the NP family, including renal NP (RNP), B-type NP (BNP) and C-type NP (CNP1, CNP3 and CNPP). To understand the functional roles of NPs in neural plastic changes, we aimed to characterize NPs and their receptors in chick brain. We found that CNP3 mRNA was expressed in several regions in the telencephalon, including the visual Wulst (VW, considered as mammalian visual cortex) and amygdala. CNP1 mRNA was expressed throughout the telencephalon. Using real-time PCR, the gene expression levels of NPs and their receptors (NPR1 and NPR2) were studied during and after the critical period of imprinting (post-hatching day [P]1 and P7). CNP3 mRNA was found to show higher expression in the VW of P1 chicks than in VW of P7 chicks. Moreover, the ability of these peptides to stimulate chicken NPR1 or NPR2 was tested in HEK293 cells expressing either of the receptors. The activation of NPR1 was stronger with CNP3 than with other subtypes of CNP. In the VW, CNP3-expressing cells were negative for NPR1, but they resided in the vicinity of NPR1-expressing cells. These results suggest that CNP3 and its receptor NPR1 in the VW may have functional roles in the early learning.

Differential projections of the densocellular and intermediate parts of the hyperpallium in the pigeon (Columba livia).

The visual Wulst in birds shows a four-layered structure: apical part of the hyperpallium (HA), interstitial part of HA (IHA), intercalated part of hyperpallium (HI), and densocellular part of hyperpallium (HD). HD also connects with the hippocampus and olfactory system. Because HD is subjacent to HI, the two have been treated as one structure in many studies, and the fiber connections of HD have been examined by afferents and efferents originating outside HD. However, to clarify the difference between these two layers, they need to be treated separately. In the present study, the fiber connections of HD and HI were analyzed with tract-tracing techniques using a combination of injections of cholera toxin subunit B (CTB) for retrograde tracing and biotinylated dextran amine (BDA) for anterograde tracing. When the two tracers were bilaterally injected in HD, a major reciprocal connection was seen with the dorsolateral subdivision (DL) of the hippocampal formation. When CTB and BDA were bilaterally injected in HI, strong reciprocal connections were found between HI and HA. Next, projection neurons in HD and HI were examined by double staining for CTB combined with vesicular glutamate transporter 2 (vGluT2) mRNA in situ hybridization. After CTB was injected in DL or HA, many neurons revealed CTB+/vGluT2+ in HD or HI, respectively. Furthermore, in situ hybridization showed that DL and HA contained neurons expressing various subunits of ionotropic glutamate receptors: AMPA, kainate, and NMDA types. These results suggest that glutamatergic neurons in HD and HI project primarily to DL and HA, respectively.

Regulation of visual Wulst cell responsiveness by imprinting causes stimulus-specific activation of rostral cells

Imprinting behaviour in chicks can be induced exclusively during a short period after hatching. During this period, visual information on the imprinting stimulus is conveyed to the visual Wulst (VW) in the telencephalon, which corresponds to the visual cortex of mammals, and then to the memory-storing region known as the intermediate medial mesopallium. These two regions are indispensable for imprinting. We previously showed that imprinting training altered the response pattern of the VW to the imprinting stimulus; however, the precise distribution of cells and the mechanism involved with this altered response remains unclear. Here we showed that a specific population of rostral VW cells responded to the imprinting stimulus by analysing the subcellular localization of Arc/arg3.1 transcripts in VW cells. GABAergic parvalbumin (PV) cells are abundant in the dorsal region of this area, and imprinting training doubled the number of activated PV-positive neurons. An injection of bicuculline, a GABA(A) receptor antagonist, in the dorsal VW disturbed the rostral distribution of responsive cells and thus resulted in a lack of imprinting. These results suggest that activated PV cells restrict VW cells response to dorsal area to form a specific imprinting pathway.

Contrast response functions in the visual wulst of the alert burrowing owl: a single-unit study.

The neuronal representation of luminance contrast has not been thoroughly studied in birds. Here we present a detailed quantitative analysis of the contrast response of 120 individual neurons recorded from the visual wulst of awake burrowing owls (Athene cunicularia). Stimuli were sine-wave gratings presented within the cell classical receptive field and optimized in terms of eye preference, direction of drift, and spatiotemporal frequency. As contrast intensity was increased from zero to near 100%, most cells exhibited a monotonic response profile with a compressive, at times saturating, nonlinearity at higher contrasts. However, contrast response functions were found to have a highly variable shape across cells. With the view to capture a systematic trend in the data, we assessed the performance of four plausible models (linear, power, logarithmic, and hyperbolic ratio) using classical goodness-of-fit measures and more rigorous statistical tools for multimodel inferences based on the Akaike information criterion. From this analysis, we conclude that a high degree of model uncertainty is present in our data, meaning that no single descriptor is able on its own to capture the heterogeneous nature of single-unit contrast responses in the wulst. We further show that the generalizability of the hyperbolic ratio model established, for example, in the primary visual cortex of cats and monkeys is not tenable in the owl wulst mainly because most neurons in this area have a much wider dynamic range that starts at low contrast. The challenge for future research will be to understand the functional implications of these findings.

Multiple Visual Field Representations in the Visual Wulst of a Laterally Eyed Bird, the Zebra Finch (Taeniopygia guttata)

The visual wulst is the telencephalic target of the avian thalamofugal visual system. It contains several retinotopically organised representations of the contralateral visual field. We used optical imaging of intrinsic signals, electrophysiological recordings, and retrograde tracing with two fluorescent tracers to evaluate properties of these representations in the zebra finch, a songbird with laterally placed eyes. Our experiments revealed that there is some variability of the neuronal maps between individuals and also concerning the number of detectable maps. It was nonetheless possible to identify three different maps, a posterolateral, a posteromedial, and an anterior one, which were quite constant in their relation to each other. The posterolateral map was in contrast to the two others constantly visible in each successful experiment. The topography of the two other maps was mirrored against that map. Electrophysiological recordings in the anterior and the posterolateral map revealed that all units responded to flashes and to moving bars. Mean directional preferences as well as latencies were different between neurons of the two maps. Tracing experiments confirmed previous reports on the thalamo-wulst connections and showed that the anterior and the posterolateral map receive projections from separate clusters within the thalamic nuclei. Maps are connected to each other by wulst intrinsic projections. Our experiments confirm that the avian visual wulst contains several separate retinotopic maps with both different physiological properties and different thalamo-wulst afferents. This confirms that the functional organization of the visual wulst is very similar to its mammalian equivalent, the visual cortex.

Features of the Retinotopic Representation in the Visual Wulst of a Laterally Eyed Bird, the Zebra Finch (Taeniopygia guttata)

The visual wulst of the zebra finch comprises at least two retinotopic maps of the contralateral eye. As yet, it is not known how much of the visual field is represented in the wulst neuronal maps, how the organization of the maps is related to the retinal architecture, and how information from the ipsilateral eye is involved in the activation of the wulst. Here, we have used autofluorescent flavoprotein imaging and classical anatomical methods to investigate such characteristics of the most posterior map of the multiple retinotopic representations. We found that the visual wulst can be activated by visual stimuli from a large part of the visual field of the contralateral eye. Horizontally, the visual field representation extended from -5° beyond the beak tip up to +125° laterally. Vertically, a small strip from -10° below to about +25° above the horizon activated the visual wulst. Although retinal ganglion cells had a much higher density around the fovea and along a strip extending from the fovea towards the beak tip, these areas were not overrepresented in the wulst map. The wulst area activated from the foveal region of the ipsilateral eye, overlapped substantially with the middle of the three contralaterally activated regions in the visual wulst, and partially with the other two. Visual wulst activity evoked by stimulation of the frontal visual field was stronger with contralateral than with binocular stimulation. This confirms earlier electrophysiological studies indicating an inhibitory influence of the activation of the ipsilateral eye on wulst activity elicited by stimulating the contralateral eye. The lack of a foveal overrepresentation suggests that identification of objects may not be the primary task of the zebra finch visual wulst. Instead, this brain area may be involved in the processing of visual information necessary for spatial orientation.

Magnetoreception systems in birds: A review of current research

At least two independent systems of magnetoreception are currently believed to exist in birds, based on different biophysical principles, located in different parts of their bodies, and with different neuroanatomical mechanisms. One magnetoreceptory system is located in the retina, and may be based on photochemical reactions on the basis of cryptochrome. Information from these receptors is processed in a specialized part of visual Wulst, the so-called Cluster N. There are good reasons to believe that this visual magnetoreceptor processes compass magnetic information necessary for migratory orientation. The second magnetoreceptory system is probably iron-based (biogenic magnetite), located somewhere in the upper beak (its exact location and ultrastructure of receptors remain unknown) and innervated by the ophthalmic branch of trigeminal nerve. It cannot be ruled out that this system participates in spatial representation and helps forming either a kind of map or more primitive signpost sense (identification of specific geographic regions), based on regular spatial variation of the geomagnetic field. The magnetic map probably enables navigation of migrating birds across hundreds and thousands of kilometres. Apart from these two systems, whose existence has been convincingly shown (even if some details are not fully clear yet), there is evidence for the existence of magnetoreceptors based on the vestibular system. It cannot be ruled out that iron-based magnetoreception takes place in lagena (a part of inner ear in fishes, amphibians, reptiles and birds), and the information perceived is processes in vestibular nuclei. The very existence of this magnetoreception system needs verification, and its function remains completely open.

BDNF mediated activity dependent maturation of visual Wulst following prenatal repetitive auditory stimulation at a critical developmental period in domestic chicks (Gallus domesticus).

The developing visual circuitry attains its mature adult pattern through the process of activity-dependent refinement in which photic stimulation plays the major role. However, auditory stimulation can also facilitate the developing visual Wulst synaptic plasticity and postnatal perceptual behavior, though the underlying mechanism is unclear. We exposed the fertilized eggs of white Leghorn chickens during incubation to either species-specific calls or no sound for varying time periods depending on the functional development of the auditory and/or visual systems. The visual evoked potential (VEP) from the Wulst was recorded at embryonic days (E) 19, 20 and posthatch days (PH) 1-3, to assess functional maturation. A significant attenuation in latencies and higher amplitudes at PH1-3 in the stimulated groups that received exposure during visual system maturation, suggest beneficial effect of auditory inputs only during critical periods. Concomitant with this, there was a significant increase in the expression of BDNF and levels of neurotransmitters GABA, glutamate, norepinephrine and serotonin from E18 only in both hemispheres of the visual Wulst. A significant inter-hemispheric difference in expression was also found in all groups. These results suggest the role of BDNF in activity driven structural and functional maturation of the visual system following prenatal repetitive auditory stimulation.

Morphology, projection pattern, and neurochemical identity of Cajal's “centrifugal neurons”: The cells of origin of the tectoventrogeniculate pathway in pigeon (Columba livia) and chicken (Gallus gallus)

The nucleus geniculatus lateralis pars ventralis (GLv) is a prominent retinal target in all amniotes. In birds, it is in receipt of a dense and topographically organized retinal projection. The GLv is also the target of substantial and topographically organized projections from the optic tectum and the visual wulst (hyperpallium). Tectal and retinal afferents terminate homotopically within the external GLv-neuropil. Efferents from the GLv follow a descending course through the tegmentum and can be traced into the medial pontine nucleus. At present, the cells of origin of the Tecto-GLv projection are only partially described. Here we characterized the laminar location, morphology, projection pattern, and neurochemical identity of these cells by means of neural tracer injections and intracellular fillings in slice preparations and extracellular tracer injections in vivo. The Tecto-GLv projection arises from a distinct subset of layer 10 bipolar neurons, whose apical dendrites show a complex transverse arborization at the level of layer 7. Axons of these bipolar cells arise from the apical dendrites and follow a course through the optic tract to finally form very fine and restricted terminal endings inside the GLv-neuropil. Double-label experiments showed that these bipolar cells were choline acetyltransferase (ChAT)-immunoreactive. Our results strongly suggest that Tecto-GLv neurons form a pathway by which integrated tectal activity rapidly feeds back to the GLv and exerts a focal cholinergic modulation of incoming retinal inputs.

Prenatal music stimulation facilitates the postnatal functional development of the auditory as well as visual system in chicks (Gallus domesticus)

Rhythmic sound or music is known to improve cognition in animals and humans. We wanted to evaluate the effects of prenatal repetitive music stimulation on the remodelling of the auditory cortex and visual Wulst in chicks. Fertilized eggs (0 day) of white leghorn chicken (Gallus domesticus) during incubation were exposed either to music or no sound from embryonic day 10 until hatching. Auditory and visual perceptual learning and synaptic plasticity, as evident by synaptophysin and PSD-95 expression, were done at posthatch days (PH) 1, 2 and 3. The number of responders was significantly higher in the music stimulated group as compared to controls at PH1 in both auditory and visual preference tests. The stimulated chicks took significantly lesser time to enter and spent more time in the maternal area in both preference tests. A significantly higher expression of synaptophysin and PSD-95 was observed in the stimulated group in comparison to control at PH1-3 both in the auditory cortex and visual Wulst. A significant inter-hemispheric and gender-based difference in expression was also found in all groups. These results suggest facilitation of postnatal perceptual behaviour and synaptic plasticity in both auditory and visual systems following prenatal stimulation with complex rhythmic music.

Flavoprotein autofluorescence imaging of visual system activity in zebra finches and mice.

Large-scale brain activity patterns can be visualized by optical imaging of intrinsic signals (OIS) based on activity-dependent changes in the blood oxygenation level. Another method, flavoprotein autofluorescence imaging (AFI), exploits the mitochondrial flavoprotein autofluorescence, which is enhanced during neuronal activity. In birds, topographic mapping of visual space has been shown in the visual wulst, the avian homologue of the mammalian visual cortex by using OIS. We here applied the AFI method to visualize topographic maps in the visual wulst because with OIS, which depends on blood flow changes, blood vessel artifacts often obscure brain activity maps. We then compared both techniques quantitatively in zebra finches and in C57Bl/6J mice using the same setup and stimulation conditions. In addition to experiments with craniotomized animals, we also examined mice with intact skull (in zebra finches, intact skull imaging is not feasible probably due to the skull construction). In craniotomized animals, retinotopic maps were obtained by both methods in both species. Using AFI, artifacts caused by blood vessels were generally reduced, the magnitude of neuronal activity significantly higher and the retinotopic map quality better than that obtained by OIS in both zebra finches and mice. In contrast, our measurements in non-craniotomized mice did not reveal any quantitative differences between the two methods. Our results thus suggest that AFI is the method of choice for investigations of visual processing in zebra finches. In mice, however, if researchers decide to use the advantages of imaging through the intact skull, they will not be able to exploit the higher signals obtainable by the AFI-method.

Repetitive auditory stimulation at a critical prenatal period modulates the postnatal functional development of the auditory as well as visual system in chicks (Gallus domesticus)

The extrinsic sensory stimulation plays a crucial role in the formation and integration of sensory modalities during development. Postnatal behavior is thereby influenced by the type and timing of presentation of prenatal sensory stimuli. In this study, fertilized eggs of white Leghorn chickens during incubation were exposed to either species-specific calls or no sound. To find the prenatal critical period when auditory stimulation can modulate visual system development, the former group was divided into three subgroups: in subgroup A (SGA), the stimulus was provided during embryonic day (E)10 to E16, in SGB E17- hatching, and in SGC E10-hatching. The auditory and visual perceptual learning was recorded at posthatch day (PH) 1-3, whereas synaptic plasticity (evident from synaptophysin and PSD-95 expression), was observed at E19, E20, and PH 1-3. An increased number of responders were observed in both auditory and visual preference tests at PH 1 following stimulation. Although a decrease in latency of entry and an increase in total time spent were observed in all stimulated groups, it was most significant in SGC in auditory preference and in SGB and SGC in visual preference test. The auditory cortex of SGC and visual Wulst of SGB and SGC revealed higher expression of synaptic proteins, compared to control and SGA. A significant inter-hemispheric and gender-based difference in expression was also found in all groups. These results indicate facilitation of postnatal behaviour and synaptogenesis in both auditory and visual systems following prenatal repetitive auditory stimulation, only when given during prenatal critical period of development.