Lecture 13 Notes PDF

The Superior Colliculus and Primary Visual Cortex The suprachiasmatic nucleus of the hypothalamus (SCN) is like the brain's clock. Neurons in the SCN have intrinsic firing frequencies of about 24 hours. Things such as sleep/wake cycles are controlled by the SCN. Light input to the SCN from the retin...

The Superior Colliculus and Primary Visual Cortex The suprachiasmatic nucleus of the hypothalamus (SCN) is like the brain's clock. Neurons in the SCN have intrinsic firing frequencies of about 24 hours. Things such as sleep/wake cycles are controlled by the SCN. Light input to the SCN from the retina resets the SCN clock to the rising and the setting of the sun. The role of the superior colliculus in the visual system A. The superior colliculus acts as a tracking mechanism; it points the eyes and head toward a visual object. B. Below is a diagram depicting the superior colliculus and its inputs from the retina: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Your Visual Field A Eyes/Retinas B E C D A D C E F B F B E D C Optic nerve Optic chiasm Optic tract D C E B F A Superior Colliculus 1. Neurons in the anterior portion of the superior colliculus respond to a visual image on the center of gaze from both eyes (but only from the contralateral visual field). 2. Neurons in the posterior portion of the superior colliculus respond to a visual image in the periphery of the visual field on the contralateral side. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 3. Neurons in the center portion (at B) in the superior colliculus receive synaptic input from retinal ganglion cells (RGCs) from both eyes that respond to light at position B in the contralateral visual field. 4. This anatomical pattern of connections is the mechanism for generation of retinotopic maps in the superior colliculus. (See Figure 10.7). C. Below is a cross section diagram of the superior colliculus showing how visual, auditory, somatosensory and motor maps are aligned with each other: Cross section through superior colliculus showing location specific multimodal responses of neurons C B B A C A Respond to sight Respond to sound Respond to touch Cause movement Eye muscle Oculomotor n. Neck muscle 1. Neurons in different layers in the superior colliculus respond to input from different sensory modalities: visual, auditory, and somatosensory. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 2. Each layer contains a map of a particular sensory modality, and these maps are all aligned (or in register) with each other. 3. The motor layer contains motor neurons, not sensory neurons. Stimulating neurons at position B in the motor layer of the superior colliculus causes the head and eyes to move to look towards location B in the visual field. a. Axons from the motor layer of the superior colliculus terminate in the oculomotor nucleus and neurons from the oculomotor nucleus project to muscles that move the eye. b. Axons from the motor layer of the superior colliculus also terminate onto motor neurons which project to muscles of the neck. D. This sensory tracking, eye pointing, subsystem is important for survival. If you hear something unfamiliar, this system will cause your head and eyes to orient towards the sound to visually identify what it was you heard. This occurs without conscious awareness. E. The existence of this visual subsystem explains (and is the reason for) a phenomena called blindsight. 1. Blindsight is “sight” that remains when a person is cortically blind. (The eyes and optic nerve work properly, but the visual cortex is damaged so that the person cannot perceive visual images). 2. A person that is cortically blind cannot “see” or identify visual objects, but can localize and track objects in visual space and they do this without awareness. 3. The superior colliculus is homologous to the optic tectum, which is the main visual brain area in non-mammalian vertebrates. Apparently, our superior colliculus became specialized for tracking and lost the ability to contribute to object identification, as that function expanded in visual cortex. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. F. You now have an example of a complete neural circuit, starting from photoreceptors in the eyes to the muscles that produce movement of the eyes and head. The cortical visual subsystem A. Thus far we have discussed two visual subsystems: 1. The superior colliculus which is involved in object tracking. 2. The suprachiasmatic nucleus, which is the brain's clock. The suprachiasmatic nucleus is located dorsal to (above) the optic chiasm. Spontaneous daily patterns of activity of neurons in this region are the reason that people experience jet lag and the reason people get over jet lag (visual input into the suprachiasmatic nucleus resets the brain's clock). B. Another visual subsystem is the cortical portion of the visual system, which is the part used in the perception of a visual stimulus. C. Below is a diagram of a side view of a human brain depicting some of the major pathways in the cortical subsystem of the visual system in a simplified form: © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Lateral Geniculate Nucleus of the Thalamus (LGN) “Dorsal stream” V5 Eyes V3 V2 V1 V4 Inferotemporal Cortex (IT) “Ventral stream” 1. Neurons in different regions of the visual cortex (V) have different functions. a. Neurons in V5 are highly sensitive to movement but not color of objects in the visual field. b. Neurons in V4 are highly sensitive to color but not movement of objects in the visual field. 2. Perception of visual objects is dependent upon activity of neurons in these cortical areas (V3, V4, V5...). For example, if a person’s V4 is destroyed then that person’s visual world will be black and white and memories are also in black or white. D. The diagram below shows the pattern of nasal and temporal hemiretina projections to the 6 layers of the lateral geniculate nucleus of the thalamus (LGN): © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. Your Visual Field A Eyes/Retinas B E F Left nasal hemiretina E D C D A C B Right temporal hemiretina B F E D C LGN (6 layers) D D D D D D E E E E E E F F F L6 L5 L4 L3 L2 L1 A A A B B B B B B C C C C C C L6 L5 L4 L3 L2 L1 Primary visual cortex 1. The basic form of projections from the retinal ganglion cells to the LGN is similar to that of the retinal ganglion cells to the superior colliculus, so that neurons in the LGN respond only to visual images in the contralateral visual field. © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed. 2. The LGN has six layers and each layer is retinotopically mapped, with neurons in each layer receiving input from the RGCs from only one hemiretina. Consequently, the neurons in the LGN are monocularly (respond to input from one eye) driven. Each of these maps are aligned with each other. 3. Why this repetition of visuotopic maps in the LGN? The cells in the different layers are processing different aspects of the visual image. 4. There are physical differences in the cells of the different layers: a. The cells in layers 1 and 2 are larger and are called magnocellular. Cells here respond to movement and are achromatic (do not respond to color). b. The cells in layers 3 through 6 are smaller and are called parvocellular. Cells here are color-sensitive and are more sensitive to detail. This is an example of parallel processing. The idea is that different aspects of a visual object (aspects like color, movement, shape) are coded by different parallel pathways. 5. Projections to the primary visual cortex (V1) from the LGN are solely ipsilateral. Therefore, neurons in V1 respond exclusively to visual input from the contralateral visual field. This means that if the right V1 is destroyed, you would be blind to input in the left visual field (but still have “blindsight” due to an intact superior colliculus). © Hongdian Yang. This content is protected and may not be shared, uploaded, or distributed.