Optical imaging of cortical activity offers several advantages over conventional electrophysiological and anatomical techniques. One can map a relatively large region, obtain successive maps to different stimuli in the same cortical area and follow variations in response over time. In the intact mammalian brain this imaging has been accomplished with the aid of voltage sensitive dyes. However, it has been known for many years that some intrinsic changes in the optical properties of the tissue are dependent on electrical or metabolic activity. Here we show that these changes can be used to study the functional architecture of cortex. Optical maps of whisker barrels in the rat and the orientation columns in the cat visual cortex, obtained by reflection measurements of the intrinsic signal, were confirmed with voltage sensitive dyes or by electrophysiological recordings. In addition, we describe an intrinsic signal originating from small arteries which can be used to investigate the communication between local neuronal activity and the microvasculature. One advantage of the method is that it is non-invasive and does not require dyes, a clear benefit for clinical applications.

We have shown previously the existence of small, activity-dependent changes in intrinsic optical properties ofcortex that are useful for optical imaging ofcortical functional architecture. In this study we introduce a higher resolution optical imaging system that offers spatial and temporal resolution exceeding that achieved by most alternative imaging techniques for imaging cortical functional architecture or for monitoring local changes in cerebral blood volume or oxygen saturation. In addition, we investigated the mechanisms responsible for the activity-dependent intrinsic signals evoked by sensory stimuli, and studied their origins and wavelength dependence. These studies enabled high-resolution visualization of cortical functional architecture at wavelengths ranging from 480 to 940 nm. With the use of near-infrared illumination it was possible to image cortical functional architecture through the intact dura or even through a thinned skull. In addition, the same imaging technique proved useful for imaging and discriminating sensoryevoked, activity-dependent changes in local blood volume and oxygen saturation (oxygen delivery). Illumination at 570 nm allowed imaging of activity-dependent blood volume increases, whereas at 600-630 nm, the predominant signal probably originated from activity-dependent oxygen delivery from capillaries.

A high spatial resolution optical imaging system was developed to visualize cerebral cortical activity in vivo. This method is based on activity-dependent intrinsic signals and does not use voltage-sensitive dyes. Images of the living monkey striate (VI) and extrastriate (V2) visual cortex, taken during visual stimulation, were analyzed to yield maps of the distribution of cells with various functional properties. The cytochrome oxidase--rich blobs of V1 and the stripes of V2 were imaged in the living brain. In V2, no ocular dominance organization was seen, while regions of poor orientation tuning colocalized to every other cytochrome oxidase stripe. The orientation tuning of other regions of V2 appeared organized as modules that are larger and more uniform than those in V1.

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The processing of sensory information, coordination of movement and other higher brain functions are carried out by millions of neurons that form elaborate networks. Individual neurons are synaptically connected to hun dreds or thousands of other neurons that shape their response properties. How these neurons and their intricate connections endow the brain with its remark able performance is one of the central questions in neurobiology.In the mammalian brain, cells that perform a given function or share common functional properties are often grouped together (e.g., the orienta tion and ocular dominance columns of the visual cortex). Attaining an under standing of the three dimensional functional organization of such groups of cells is a key step towards revealing the mechanisms of information process ing in a given cortical region. Thus, especially promising are experimental methods that allow the visualization of the functional organization of a cortical region, particularly methods that provide high spatial and temporal resolution. Currently there is a surge of interest in several imaging techniques that yield information about the spatial distribution of active neurons in the brain. These methods include the 2-deoxyglucose method (2-DG), radioactive imaging of changes in blood flow, electroencephalography, magnetoencepha lography, positron emmision tomography (PET), nuclear magnetic resonance imaging (MRI), and thermal imaging. Each technique has advantages as well 543 0066-4278/89/0315-0543$02.00 Annu. Rev. Physiol. 1989.51:543-559. Downloaded from www.annualreviews.org Access provided by University of Central Florida on 02/04/15. For personal use only. Quick links to online content Further ANNUAL REVIEWS 544LlEKE ET AL as significant limitations, and most techniques still suffer from either limited spatial resolution, temporal resolution, or both. Optical imaging of cortical activity is an attractive technique for providing new insights to both the organization and function of the mammalian brain. Among its advantages over other methodologies are (a) the direct recording of the summed intraceIIular activity of neuronal populations, including the detection of subthreshold synaptic potentials in fine neuronal processes and tenninals, (b) the imaging of spatio-temporal patterns of activity of neuronal populations with submillisecond time resolution in vitro and in the living brain, and (c) the possibility of repeated measurements from the same cortical region with different experimental or stimulus conditions over an extended time. However, unlike some of the other imaging techniques, optical imaging is limited to exposed surfaces of the brain up to a depth of 0.5-�2 mm.

Voltage-sensitive dyes and activity-dependent intrinsic optical signals were used to study the spatio-temporal activity in the antennal lobes of honeybees. The intrinsic signals are somewhat slower than the dye signals but show a 10-fold larger intensity change. These intrinsic signals consist of at least two components--one is wavelength-independent and the other strongly wavelength-dependent, with a maximum at approximately 500 nm. Local inhibitory connections within the antennal lobes were examined by recording the activity elicited by an electrical stimulus to the antennal nerve of a slice preparation before and after applying picrotoxin to manipulate GABAergic inhibitory synapses. The inhibition starts with a delay of approximately 10 ms after onset of the response and has at least two components. The spatial distribution of the inhibition is extremely inhomogeneous, with areas of small inhibition adjacent to areas of large inhibition. Thus inhibitory interactions in the antennal lobes are not evenly distributed among the glomerular organization. Stimulation of an in vivo preparation with an odour yields a spatially restricted activity. However, the spatial map appears highly dynamic in time because the size of the activated area is a function of the time during and after the stimulus.