Neuroscience for Kids - The Eye and Its Connections
The cornea and lens bend or refract light rays as they enter the eye, in order to .. the Eye and its Connections activities and Background material will help to meet Human beings can detect a tremendous range of visual and olfactory stimuli. Abstract: In fish, axons that originate in the olfactory bulb innervate the retina and increase luminance sensitivity. In this issue of Neuron, Esposti. Its nerve fibres arise in the olfactory epithelium of the nasal chamber. It enters Its nerve fibres originate in the retina of the eye and combine to form the optic nerve. Two optic nerves meet at the floor of the diencephalon where they appear to.
Retina layer of tissue on the back portion of the eye that contains cells responsive to light photoreceptors Sclera tough, white outer covering of the eyeball; extraocular muscles attach here to move the eye 4. The retina originates from the brain and contains photoreceptors for detecting light The eye is formed during embryonic development by a combination of head ectoderm and neural tube tissue, the latter forming the retina.
Thus, the retina is not a peripheral sensory organ like skin touch receptors or taste buds on the tongue, but rather it is an outgrowth of central nervous tisse. Because of this origin, the retina has layers of neurons, internal circuits, and transmitters characteristic of the brain: The photoreceptors in the retina are of two types: These cells are actually specialized neurons that detect light. Embedded in stacks of cell membranes in the distal portions of rods and cones are molecules that absorb certain wavelengths of light.
These molecules are called photopigments and are composed of two parts: The chromophore, which is embedded in the opsin, absorbs light; in so doing it undergoes a shape change. This shape change in turn activates the opsin, setting off a cascade of events that leads to a change in the electrical state of a rod or cone cell membrane.
This change in the rod or cone cell membrane is then conducted via the rod or cone axon to other neurons in the retina, and from there to the brain. Rods function in dim light In dim light, we use our rods, which cannot work in bright light. Rods outnumber cones million rods and about 6 million cones in each retina and they amplify a light signal much more than cones. Scientists have demonstrated that absorption of even a single quantum or photon of light can trigger a chromophore shape change in one molecule of rhodopsin in a rod, leading to signal transmission.
For transmission to occur, this initial tiny event must be amplified: Cones, on the other hand, must each absorb hundreds of photons in order to send signals.
Olfaction and vision meet in the retina.
Another retinal mechanism that helps us to see in dim light or to see a tiny amount of light in the dark is the convergence of rod cell signals onto other retinal neurons. Many rods up to synapse onto the same target neurons, where the signals are pooled and reinforce one another, increasing the ability of the brain to detect a small amount of light. A synapse is a contact between a neuron and another cell where an electrochemical signal [most commonly] is transmitted to the second cell.
This convergence amplifies weak signals, but spatial resolution is lost because rod responses are averaged. That is, we cannot see fine detail using rods. In order for our eyes to make the transition to dim light, rods must adapt after being saturated with light in brighter conditions. Dark adaptation of rods takes seven to ten minutes: Other changes also occur in adaptation to dark or dim conditions, including enlarging or dilating of the pupil, which is controlled by the autonomic nervous system.
Cones mediate day vision Our vision in bright or moderate light is completely mediated by cones, which provide color vision, black and white vision, and high acuity, the ability to discern fine detail.
Like rods, cones contain an opsin and the chromophore cis-retinal, but the opsins differ from rhodopsin so that each cone is responsive to one of three colors: Cones are spread throughout the retina but are especially concentrated in a central area called the macula. At the center of the macula is the fovea, where only cones no rods are found, and these are densely packed.
When we want to read or inspect fine detail, we move our heads and eyes until the image of interest falls onto the fovea. Because the fovea lacks rods, it is easier to see in dim light by looking to the side of an object instead of directly at it. You can test this by looking to the side of a faint star so that its image falls on rods, rather than on the fovea where it probably will not register.
When you look directly at the faint star, it disappears. In contrast to the wiring of rods, only a few cones converge onto other retinal neurons to average their signals, so cones provide better spatial resolution.
In fact, each cone in the fovea synapses onto only one neuron in the next relay in the retina. This gives this area the ability to transmit fine detail, such as we use in reading. Thus, cones mediate day vision and rods take over in dim light and at night. Both rods and cones can operate at the same time under some conditions: This is why we can see the colors of neon lights on dark nights.
Visual information travels from retinal ganglion cells to the brain After converting light into electrical signals in their cell membranes, rods and cones transmit this information to other neurons in internal circuits in the retina for processing.
From these cells, messages go to the final retinal station, the ganglion cells, whose axons exit the eyeball at the optic disc and form the optic nerve, which contains about one million axons.
Because all the nerve fibers converge at the optic disc, no rods or cones are in this area and it forms a "blind spot" on the retina: Within the optic nerve, a defined group of axons from each eye crosses over to join the opposite optic nerve at the optic chiasma see Figure 3so each side of the brain receives visual information from both eyes. After the chiasma, retinal axons go to one of three areas: The information going to the midbrain does not reach conscious levels but rather produces pupillary reflexes which are controlled by the autonomic nervous system and eye movements.
In the thalamus, ganglion cell axons transmit signals to neurons in the lateral geniculate nucleus LGN where information is processed and then carried by LGN axons to the primary visual cortex in the occipital lobe of the cerebrum. These cortical cells then send messages to other "higher" cortical areas. Figure 3 shows the anatomy of this system the midbrain areas are not shown here.
The visual pathway 8. We have an area of central or focused vision and an area of peripheral vision within our fields of vision The visual field is defined as the view seen by the two eyes while looking straight ahead Figure 4. Without moving eyes or head, a person can see details well enough to read within a limited angle drawn from a point between the eyes on the forehead and two experimentally determined points to the left and right in front of the viewer, at proper focal distance.
In addition to this area of clear or central vision, we can see objects and movements to the sides of our heads, although as the distance around to the sides increases, it becomes more difficult to identify objects.
The area of central vision includes objects whose images fall onto the central area of the retina, the macula, and especially the fovea defined above. Cones in all other areas of the retina are in the periphery, and while they convey visual information, they do not provide the resolving power of the densely packed fovea. Complete visual field and central visual field, looking down onto the head. The complete visual field is the entire area in front of the eyes from the end of one lateral dashed line to the other including the central visual field.
In addition to speaking of a central and a peripheral field of vision, we can divide these fields by a vertical line down the middle into right and left visual fields. The receptive field size increases at successive processing stages in the visual pathway and, at each processing stage, it increases with the distance from the point of fixation eccentricity.
Retinal ganglion cells located at the center of visionin the foveahave the smallest receptive fields and those located in the visual periphery have the largest receptive fields. The large receptive field size of neurons in the visual periphery explains the poor spatial resolution of our vision outside the point of fixation other factors are photoreceptor density and optical aberrations.
To become aware of the poor spatial resolution in our retinal periphery, try to read this line of text while fixating your eyes in a single letter. The letter that you are fixating is being projected at the center of your fovea where the receptive fields of retinal ganglion cells are smallest.
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The letters that surround the point of fixation are being projected in the peripheral retina. You will notice that you can identify just a few letters surrounding the point of fixation and that you need to move your eyes if you want to read the entire line of text.
Modern studies have expanded the term receptive field to include a temporal dimension. The spatiotemporal receptive field describes the relation between the spatial region of visual space where neuronal responses are evoked and the temporal course of the response.
Direction selective neurons respond to some directions of movement better than others. For example, a neuron may respond to a vertical line moving leftwards but not moving rightwards.
The direction selective neurons generate visual responses with different time delays at different regions of the receptive field. Some regions respond faster to visual stimuli than others. As a consequence of these differences in response timing, a line moving from a slow to a fast region generates a stronger response than a line moving from a fast to a slow region.
Receptive field - Scholarpedia
When the line moves in the optimal direction, the slow region, which is stimulated first, responds approximately at the same time as the fast region, which is stimulated later. Visual receptive fields are sometimes described as 3-dimensional volumes in visual space to include depth in addition to planar space. On-center and Off-center receptive fields. The receptive fields of retinal ganglion cells and thalamic neurons are organized as two concentric circles with different contrast polarities.
On-center neurons respond to the presentation of a light spot on a dark background and off-center neurons to the presentation of a dark spot on a light background. Neurons at different stages in the visual pathway have receptive fields that differ not only in size but also in structure.
The complexity of the receptive field structure, just as the receptive field size, increases at successive stages of the visual pathway. Most neurons in the retina and thalamus have small receptive fields that have a very basic organization, which resembles two concentric circles. This concentric receptive field structure is usually known as center-surround organization, a term that was originally coined by Kuffler On-center retinal ganglion cells respond to light spots surrounded by dark backgrounds like a star in a dark sky.
Off-center retinal ganglion cells respond to dark spots surrounded by light backgrounds like a fly in a bright sky. In primary visual cortexreceptive fields are much more diverse and more complicated than in the retina and thalamus.
Only a few cortical receptive fields resemble the structure of thalamic receptive fields, while others have elongated subregions that respond to either dark or light spots, others respond similarly to light and dark spots through the entire receptive field and others do not respond to spots at all.
Hubel and Wiesel provided the first characterization of receptive fields in primary visual cortex and the first classification of cortical cells based on their receptive field structures. Some cortical cells respond to light and dark spots in different subregions of the receptive field and the arrangement of these subregions can be used to predict the responses of the cell to visual stimuli such as lines, bars or squared shapes.
Cells with separate subregions that respond to either light or dark spots are called simple cells. Receptive fields of four primate V1 neurons 9oo eccentricity. The receptive field of each neuron was mapped with light spots continuous lines, top panels and dark spots dotted lines, bottom panels. Unlike complex cells c,dsimple cells a,b respond to light and dark spots in different regions of the receptive field Figure taken from Chen et al. Since Hubel and Wieselother methods to classify cortical receptive fields have been proposed.
However, to this date, no classification method has been widely adopted by the entire scientific community. Among all the classification methods after Hubel and Wiesel, the one that has been most widely used is based on the responses of cortical neurons to sinusoidal drifting gratings. Some cortical neurons respond to the sinusoidal changes in luminance by generating a rectified sinusoidal response which is a rough linear replica of the stimulus while others respond by increasing the mean firing rate.
A quantitative measurement of response linearity can be obtained by Fourier analysis. Response linearity is bimodally distributed Skottun et al.
The great diversity of receptive fields in primary visual cortex makes it difficult to correlate neuronal classes with receptive field properties, as is currently possible in the retina e. Neurons in primary visual cortex can respond selectively to different attributes of the visual scene such as line orientation, direction of movement, luminance contrast, stimulus velocity, color, retinal disparity and spatial frequency frequency of black and white stripes in a degree of visual space.
Linear and non-linear V1 neurons in primate. The visual responses of linear neurons top row resemble a rectified replica of the sinusoidal stimulus drifting grating. In contrast, the visual responses of nonlinear neurons bottom row resemble a step function. Cartoon illustrating the changes in the amplitude of the stimulus continuous lines and response dotted lines across time.
Raster plots top panels and peri-stimulus time histograms PSTHs, bottom panels for the four same cells illustrated in Figure 3 Figure 3 a: Each tick in the raster plot represents a spike. Most neurons in primary visual cortex respond to moving lines and are selective to line orientation.
Some neurons are sharply tuned to orientation and fail to respond to lines that are just a bit tilted from their preferred orientation while other cortical neurons are broadly tuned and respond to a broad range of orientations. The selectivity of each neuron to line orientation and other parameters is determined to a great extent by the receptive field structure.
A very active area of research aims to build realistic models of receptive field structures that can explain neuronal responses to different stimuli. The most successful models to date were built for neurons at the earliest stages of the visual pathway.
For example, the receptive fields of retinal and thalamic neurons can be modeled quite accurately with a difference of Gaussians DOGRodieck, More complex models combine multiple functions to accurately reproduce the response of a neuron to different stimuli. These receptive field models aim to provide information about all possible stimuli that would best drive neuronal responses e.
Orientation tuning in V1 neurons. Polar plots of two neurons with sharp a and broad b orientation tuning measured with drifting sinusoidal gratings. The radial coordinate illustrates firing rate; the angle illustrates the direction of movement. The PSTHs show neuronal responses to gratings drifting for one second in four different directions of movement. Scale bars refer to the radius of each polar plot.
The receptive field size of neurons in primary visual cortex depends strongly on the stimulus contrast. The size can be more than two times larger when measured with low contrast stimuli than when measured with high contrast stimuli. Neurons in higher cortical areas have large receptive fields and can be more selective to the identity of the stimulus than to its physical location.
Somatosensory receptive fields The receptive fields of somatosensory neurons share much in common with the receptive fields of visual neurons.
As for visual neurons, the somatosensory receptive fields comprise a restricted 2-dimensional region of space where a stimulus can evoke a neuronal response. In somatosensory neurons, however, space refers to a region of the body and the stimulus can be touch, vibration, temperature or pain.
Similar to visual neurons, the receptive fields of somatosensory neurons are smaller in the regions of the body where the perceptual spatial resolution is highest. The fingertips have the highest spatial resolution and the smallest receptive fields while the thigh and calf region have the lowest spatial resolution and largest receptive fields. The spatial resolution to light-touch stimulation can be evaluated by measuring two-point discrimination thresholds. The subject has to report whether the skin is touched either with one or two pointy objects that are closely spaced.
When the distance between the two objects is small, it is not possible to reliably distinguish between one or two objects touching. The minimum distance that is required to distinguish two pointy objects is called the two point discrimination threshold. The two point discrimination threshold is less than 5 mm at the finger tips and is about 40 mm at the thigh.
As in the visual system, the receptive fields in the somatosensory thalamus have center-surround organization and those in the somatosensory cortex have more complex receptive field structures that make the neurons selective to the orientation and direction of motion of a stimulus.
Auditory receptive fields The auditory sensory epithelium responds selectively to the sound frequency and not to the spatial location of the stimulus as is the case in the visual and somatosensory systems. Consequently, it is sound frequency that defines the auditory receptive fields at the earliest stages of sensory processing. Whereas in the visual and somatosensory systems the spatial receptive fields are constructed directly from connections originating in the sensory epitheliums receptors in retina and skinin the auditory system, the spatial receptive fields have to be synthesized by specific circuits that compare differences in stimulus intensity and timing between the two ears.
Therefore, in auditory physiology, the term receptive field is frequently used with two different meanings. As a first meaning, an auditory receptive field can refer to the range of sound frequencies that most optimally stimulate the neuron auditory spectrotemporal receptive field. As a second meaning, an auditory receptive field can refer to the region in auditory space where a stimulus can evoke a neuronal response auditory spatial receptive field.
The sensory organ in the auditory system, the cochleahas a precise representation of sound frequency, which is organized like a piano scale: Neurons at different stages of the auditory pathway can be very sensitive to small variations in sound frequency and their responses can have different time courses. Both the frequency range and time course of the response are quantitatively represented in the spectrotemporal receptive field.
In contrast, the auditory spatial receptive field resembles more the visual and somatosensory receptive fields in that it represents an area of space where a stimulus sound generates a neuronal response. Like with visual and somatosensory receptive fields, spatial receptive fields at early stages in the auditory pathway have center-surround organization. For example, some auditory neurons in the midbrain respond to sounds presented at a defined region of auditory space, which is the receptive field center, and the response is reduced when the stimulus is presented in a region surrounding the center, which is the receptive field surround.
The center-surround receptive fields of auditory neurons cover a much larger region of space than visual and somatosensory receptive fields with similar center-surround organization. Olfactory receptive fields The receptive fields of olfactory neurons have been much less studied than the receptive fields of neurons in other sensory systems. The olfactory receptive fields are particularly difficult to characterize because the odor parameters that define the olfactory space are poorly known.
Recent evidence indicates that olfactory receptive fields are mapped along the dimension of molecular carbon chain length. The receptive fields of cells at early stages of olfactory processing often include inhibitory surrounds to the longest and shortest effective chain lengths Wilson, Glossary Auditory spatial receptive field: The region of space where a sound can generate a response in an auditory neuron.
Auditory spectrotemporal receptive field: Spectrum of sound frequencies that generate a response in an auditory neuron represented as a function of the time-course of the response. Refers to neurons that respond similarly to wide range of variations within a given stimulus dimension. For example, neurons that have broad orientation tuning respond similarly to all line orientations Figure 5 b.
Neurons that have broad spatial frequency tuning respond similarly to a wide range of spatial frequencies. A portion of the inner ear which is a spiraled, hollow, conical chamber of bone.
The auditory sensory neurons are located inside the cochlea.