Lecture 7 Flashcards
History
Little discussion in monotheism faiths, not a lot of discussion about the nature of perception. Buddhism did focus on this though - perceptual relationship between person and the world, don’t get that in Western religion. Philosophy - Descartes was the first person to think about observer and observed. Conclusion that it’s all an illusion. Kant - noumenal (what reality is) and phenomenal (your creation of it) world. It’s not the world, it’s your recreation. Schobenhauer - world is my representation. Berkeley - sylopsism - nothing in the world outside of perception, world is a fabrication of your mind.
The nature of sensation and perception
The only input our brains perceive from the real world is a series of action potentials passed along neurons of sensory pathways. How nerves turn energy (light, sound, touch) into nerve impulses - sensory transduction, is understood. The pathways those nerves take to reach the brain are known. Less is know about how we generate an experience of perception from those impulses. Sensation: the registration of physical stimuli from the environment by sensory organs. Perception: subjective interpretation of sensation by the brain. Our visual experience is not an objective reproduction of what’s out there but a subjective construction of reality manufactured by the brain. Sensory systems have a restricted range of responsiveness (e.g. the frequency range for hearing varies across species). Species evolve different sensory abilities in response to ecological pressures. The construction of reality is a precise representation, there is precision between what you see and what you perceive.
Sensory transduction and generator potentials
An adequate stimulus is the type of stimulus to which a sensory organ is particularly adapted. Sensory transduction - the conversion of sensory energy from an adequate stimulus into a local change of membrane potential in a sensory receptor cell (i.e. a generator potential). Generator potentials resemble EPSPs. If the generator potential exceeds the firing threshold, action potentials are generated that travel via the sensory nerves.
Labeled lines
All sensory information from all systems is encoded by action potentials that travel along peripheral nerves to the CNS. How do action potentials code different kinds of sensation (like vision vs touch)? Labeled lines. Each nerve input to the brain reports only a specific type of information. Labeled lines occur both across different sensory systems and within specific sensory systems. There is a 1:1 mapping between sensitive parts of the body and receptivity in the brain. Representation in the brain of particular area of finger, representation of neurons, precise neurons so you know where things are coming from. There’s coherence between peripheral and central neurons. Push vs cut on finger activates different things. Tractography.
Sensory cortices
Highly organized. Primary sensory cortex - exists for each modality, first cortical area receiving input for each modality. Nonprimary or secondary sensory cortex - receives main input via direct projections from the primary sensory cortical area for that modality.
The thalamus
Central relay station of information to the cortex for most senses. Information from each sensory modality sent to a different area of the thalamus. Cortical modulation of sensory information: brain can modulate information it receives - inhibit activity in ascending sensory neurons (like pain). In the CNS, the cortex can direct the hypothalamus to suppress some and emphasize other information. Thalamus is also implicated in sensory and attention allocation and emotion.
Topographical organization
Neurons at all levels of visual and touch pathways - from surface sheet of receptors to visual cortex are arranged in an orderly, map-like manner (labeled lines). Primary somatosensory cortex involves a somatotopic map that is a spatially organized neural representation of the body. Vision involves a similar retinotopic map. Topographical organization: location of peripheral sensory neuron maps directly onto specific cells of the cortex. There’s a somatotopic map of the body in the brain.
Intensity
Range fractionation - specialized cells sensitive to varying levels of intensity - as strength of stimulus increases, additional sensory neurons are recruited. Multiple neurons can act in parallel - as the stimulus strengthens, more neurons are recruited. A single neuron can convey stimulus intensity by changing the frequency of its action potentials.
Sensory receptor adaptation
Receptors have tonic or phasic response properties. Tonic receptors: show slow or no decline in action potential frequency to a maintained stimulus. Ex: pain receptors. Phasic receptors: display adaptation and decrease frequency of action potentials to a maintained stimulus. Olfactory and tactile receptors. Adaptation - the progressive loss of receptor sensitivity as stimulation is maintained. Many sensory systems emphasize change in the external world, which is most relevant for behavior and survival.
Phasic receptors display adaptation
The neuron fires rapidly when the stimulus is first applied (regardless of intensity) and then it adapts. Tonic receptors don’t adapt, pain doesn’t go away.
The task of visual perception
Vision is our primary sensory experience (primates). About 1/3 of the human cerebral cortex is devoted to visual analysis and perception. We perceive reality at a level of analysis that was adaptive to our ancestral survival. Direct motion within range of speed appropriate to the locomotion of animals (bullet too fast and clock too slow). We perceive objects as solid despite insights of quantum mechanics. In a fraction of a second, our visual system must: capture light energy of different wavelengths reflecting off objects in all directions (the eye). Transduce this light into a 2D array of neuronal activity (photoreceptors in the retina). Process visual information, perceive important visual features such as shape, color, motion, location distance, and recognize identity of objects in our 3D dynamic world (Other retinal neurons, visual pathways, and the cerebral cortex).
Eye and retina
The eye is an elaborate structure with optical functions (capturing light and forming detailed spatial images) and neural functions (transducing light into neural signals and processing those signals).
Structures of the human eye
Cornea - biconvex clear layer that refracts light (forming a backward, inverted image on the light-sensitive retina). Iris - colored part of the eye that regulates the amount of light reaching the retina (opens - sympathetic and closes - parasympathetic to allow more or less light in through an opening called the pupil, Ach antagonist for eye dilation). Lens - focuses light (ciliary muscles adjust the curvature of the lens to focus on near or far objects (accommodation, farshigtedness with age of lens and loss of flexibility). Retina - thin layers of neurons on the back of the eye. Where light energy initiates neural activity (visual transduction). Fovea - central region of the retina - highest density of photoreceptors (cones only), so sharpest visual acuity. Optic disk - where axons of the optic nerve leave the eye. Forms a blind spot due to lack of photoreceptors. Extraocular muscles - precisely control eye movements. Three pairs of muscles (innervated by cranial nerves III, IV, VI), superior colliculus.
Focusing images on the retina
In normal vision: refraction bends the light so that the rays converge, bringing the image into sharp focus on the retina. Myopia (nearsightedness): difficulty seeing distant objects and develops if the eyeball is too long, causing images to form in the front of the retina. Myopia correction: a concave lens causes rays to initially diverge so that the images fall on the retina.
Retina and fovea
Retina: light sensitive surface at the back of the eye consisting of photoreceptor cells (rods and cones) and other neurons. Translates light into action potentials (transduction). Fovea - region at the center of the retina that is specialized for high acuity and color vision. Receptive field at the center of the eye’s visual field (point of fixation). Receives direct light input that does not pass through other cells or blood vessels.
Anatomy of retina
Photoreceptor cells - rods and cones. Bipolar cells - receive input from photoreceptors (rods, cones, and bipolar cells involved local, graded potentials). Neural signals in retina converge on retinal ganglion cells whose axons give rise to optic nerves (ganglion cells generate action potentials). From receptor cells to ganglion cells, a huge amount of information is compressed, over 100 million receptor cells are compressed into 1 million ganglion cell action potentials. There’s more consolidation with rods than with cones, rods need less detailed acuity.
Receptors
Rods have maximal absoprtion at 496nm but don’t contribute to color perception. Three types of cone pigments absorb light over a range of wavelengths, but their maximal absoprtions are at 419nm (blue/short), 531 nm (green/middle), and 559nm (red/long). 400-700nm is human eye. Night goggles let you see infrared. Cones are central to color perception. Cones have sensitivity peaks - but they respond to input across the spectrum, this is what creates all the hues you can see. Brain generates perception of color, brain doesn’t differentiate between sensitivity and color perceptions.
Scotopic/Photopic
Rods and cones correspond to two systems. Scotopic/rods allows for night vision (high convergence as info from many rods converge onto each scotopic retinal ganglion cell - large receptive fields). High sensitivity (can see in dim light) but low acuity. 100 million rods converge on 500,000 retinal ganglion cells. Takes time to adjust to the dark after the light. Photopic/cones: allows color vision - little convergence as photopic retinal ganglion cells receive input from only one or a few cones - small receptive fields. Low sensitivity - requires bright light, but high acuity (sharp vision).
Stimulation hyperpolarizes rods and cones
Phototransduction: light particles (quanta or photons) strike the discs and are captured by photopigment receptor molecules. A cascade of events produces hyperpolarization of rods and cones. Disc structure of rods and cones contribute to their sensitivity. Bipolar cells use hyperpolarization as indicator of low light.
Phototransduction
Photoreceptors are depolarized in the dark and constantly producing glutamate. Light triggers hyperpolarization of the cell, so it releases less neurotransmitter. Graded release: the magnitude of hyperpolarization determines the reduction in neurotransmitter release (no action potentials).
Retinal ganglion cells to optic nerve
Neural signals in the retina converge on RGCs, the only retinal cells that generate action potentials. RGC axons form the optic nerve which sends information to the brain along several pathways. Hyperpolarization and depolarization of rods and cones, excitation and inhibition of bipolar cells. Bipolar cells help consolidate rod and cone input. They have local and graded potentials and do the APs. Axons of ganglion cells go through optic disc and into brain. 30% of brain devoted to vision.
Four visual pathways
Geniculostriate system (conscious vision): retina to LGN of the thalamus to striate cortex (V1, looks striated because of layers) to other visual areas. V1 is central to processing vision. Tectopulvinar system (saccades, attentional orienting): retina to superior colliculus of the tectum to pulvinar of the thalamus, to other visual cortical areas. Retinohypothalamic pathway (circadian rhythms): retina to SCN of hypothalamus. Pupillary pathway (pupil reflex): retina to endiger-westphal nucleus.
Geniculostriate system
Conscious vision. Topographic projection or retinotopic mapping: a mapping that preserves the point to point correspondence between neighboring parts of space. For example, the retina extends a topographic projection onto the cortex.
Extrastriate cortex
Occipital lobe is composed of at least six different visual regions. Primary visual cortex: V1, striate cortex. Receives primary input from the LGN. Secondary visual cortex (V2-V5 - extrastriate cortex): visual cortical areas outside of the striate cortex. Ex: V4 in color, V5 in motion. Different parts of brain serve different inputs and challenges. Stroke in V5 means can’t see motion. Different nodes are solving/serving different problems. Consciousness puts it all together.
Disorders from extrastriate cortex injury
Patient J1 - V4 lesion - lost perception of color, could no longer dream in color, couldn’t even imagine color. V5 lesion: loss of motion perception, fluid appeared frozen and appeared to jump when poured into a container, individuals in the environment seemed to disappear and then reappear in a different location. Difficulty tracking conversations because could not see movement of speaker’s lips.
Dorsal and ventral visual streams
Two main processing streams originate in visual cortex: dorsal - projects to vision-related areas of the parietal cortex, for assessing the location of objects (where) and guiding our movement toward or away from them - there where or how pathway (how = being guided towards objects). Ventral: projects to vision-related areas of the temporal cortex. For identifying objects - what, what pathway identifies what an object is. Hippocampus is in ventral stream.
Prosopagnosia
Face-responsive region of the right inferior temporal cortex called the fusiform face area. Prosopagnosia is inability to engage this area to recognize faces. Some individuals have congenital prosopagnosia. Then there’s prosopagnosia secondary to inferior temporal damage. Unable to recognize familiar faces but can describe facial characteristics and identify objects. Can recognize familiar people from nonfacial cues (like voice).
