A canonical interlaminar circuit in the sensory dorsal ventricular ridge of birds: The anatomical organization of the trigeminal pallium.

The dorsal ventricular ridge (DVR), which is the largest component of the avian pallium, contains discrete partitions receiving tectovisual, auditory, and trigeminal ascending projections. Recent studies have shown that the auditory and the tectovisual regions can be regarded as complexes composed of three highly interconnected layers: an internal senso-recipient one, an intermediate afferent/efferent one, and a more external re-entrant one. Cells located in homotopic positions in each of these layers are reciprocally linked by an interlaminar loop of axonal processes, forming columnar-like local circuits.

Parallel organization of the avian sensorimotor arcopallium: Tectofugal visual pathway in the pigeon (Columba livia).

The sensory-motor division of the avian arcopallium receives parallel inputs from primary and high-order pallial areas of sensory and vocal control pathways, and sends a prominent descending projection to ascending and premotor, subpallial stages of these pathways. While this organization is well established for the auditory and trigeminal systems, the arcopallial subdivision related to the tectofugal visual system and its descending projection to the optic tectum (TeO) has been less investigated. In this study, we charted the arcopallial area displaying tectofugal visual responses and by injecting neural tracers, we traced its connectional anatomy.

Intratelencephalic projections of the avian visual dorsal ventricular ridge: Laminarly segregated, reciprocally and topographically organized.

Recent reports have shown that the avian visual dorsal ventricular ridge (DVR) is organized as a trilayered complex, in which the forming layers-the thalamo-recipient entopallium (E), an overlaying nidopallial stripe called intermediate nidopallium (NI), and the dorsally adjacent mesopallium ventrale-appear to be extensively interconnected by topographically organized columns of reciprocal axonal processes running perpendicular to the layers, an arrangement highly reminiscent to that of the sensory cortices of mammals. In the present report, we implemented in vivo anterograde and retrograde tracing techniques aiming to elucidate the organization of the connections of this complex with other pallial areas. Previous studies have shown that the efferent projections of the visual DVR originate mainly from the NI and E, reaching several distinct associative and premotor nidopallial areas.

Attentional capture? Synchronized feedback signals from the isthmi boost retinal signals to higher visual areas.

When a salient object in the visual field captures attention, the neural representation of that object is enhanced at the expense of competing stimuli. How neural activity evoked by a salient stimulus evolves to take precedence over the neural activity evoked by other stimuli is a matter of intensive investigation. Here, we describe in pigeons (Columba livia) how retinal inputs to the optic tectum (TeO, superior colliculus in mammals), triggered by moving stimuli, are selectively relayed on to the rotundus (Rt, caudal pulvinar) in the thalamus, and to its pallial target, the entopallium (E, extrastriate cortex).

A cholinergic gating mechanism controlled by competitive interactions in the optic tectum of the pigeon.

We describe the operation of a midbrain neural circuit in pigeons that may participate in selecting and attending to one visual stimulus from the myriad displayed in their visual environment. This mechanism is based on a topographically organized cholinergic signal reentering the optic tectum (TeO). We have shown previously that, whenever a visual stimulus activates neurons in a given tectal location, this location receives a strong bursting feedback from cholinergic neurons of the nucleus isthmi pars parvocellularis (Ipc), situated underneath the tectum.

Oscillatory bursts in the optic tectum of birds represent re-entrant signals from the nucleus isthmi pars parvocellularis.

Fast oscillatory bursts (OBs; 500-600 Hz) are the most prominent response to visual stimulation in the optic tectum of birds. To investigate the neural mechanisms generating tectal OBs, we compared local recordings of OBs with simultaneous intracellular and extracellular single-unit recordings in the tectum of anesthetized pigeons. We found a specific population of units that responded with burst discharges that mirrored the burst pattern of OBs.

The mapping of the visual field onto the dorso-lateral tectum of the pigeon (Columba livia) and its relations with retinal specializations.

Most of the physiological studies of the pigeon retino-tectal visual pathway have investigated the accessible tectum, a small dorso-lateral tectal section that can be easily accessed by a simple craniotomy. However, at present we lack a detailed study of the topographical arrangement between the visual field, the retina and the accessible tectum. In particular, it is not known which section of the visual field is mapped onto the accessible tectum, and which of the specialized retinal areas mediates this projection.

Spatial organization of the pigeon tectorotundal pathway: an interdigitating topographic arrangement.

The retinotectofugal system is the main visual pathway projecting upon the telencephalon in birds and many other nonmammalian vertebrates. The ascending tectal projection arises exclusively from cells located in layer 13 of the optic tectum and is directed bilaterally toward the thalamic nucleus rotundus. Although previous studies provided evidence that different types of tectal layer 13 cells project to different subdivisions in Rt, apparently without maintaining a retinotopic organization, the detailed spatial organization of this projection remains obscure.