Abstract:
A system for tracking the laboratory animal position and movement in a walled enclosure or cage for observation and evaluation is disclosed. The system consists of a plate placed on the bottom of the cage whereon multiple electrodes are configured as column-row two-dimensional electrode array, an electronic circuit detecting and measuring the capacitance between said electrodes, and a microprocessor determining the animal&#39;s location. The electronic circuit repeatedly measures the capacitance between the electrodes in a sequential manner. The animal&#39;s location and movement is determined by detecting the changes in capacitance on said plate.

Description:
BACKGROUND OF THE INVENTION 
     Locomotion function is one of the important behavior parameters in animal research for human neurodegenerative diseases such as Parkinson&#39;s disease, Huntington&#39;s disease, and Alzheimer&#39;s disease. Neurodegenerative animal models have been well-established in rodents. Animal models with such diseases exhibit characteristic motoric deficits including declined movement activity, decreased movement speed, and reduced traveling distance. With an effective drug treatment, the animal locomotion function could be recovered to a great extent. Therefore, automated logging of the animal&#39;s locomotion function is essential in the pharmaceutical laboratory. 
     A number of inventors proposed methods to detect laboratory animal dynamic motion activity. The Stigmark et al U.S. Pat. No. 3,656,456 provides a system to monitor motion activity by detecting electrode capacity imbalance across a transformer bridge which results from animal movement in the environment. The Castaigne U.S. Pat. No. 3,540,413, the Vajnoszky U.S. Pat. No. 3,633,001 and the Meetze U.S. Pat. No. 3,974,798 disclose methods to detect laboratory animal motion activities by measuring the conductance of animals in contact with electrodes. 
     Methods of detecting laboratory animal locations are also provided by many other inventors. The earlier method disclosed by U.S. Pat. No. 3,304,911 (Hakata, et al) uses a pair of movable infrared light receivers to track animal locations in a square field. Salmons U.S. Pat. No. 3,439,358 utilizes multiple receiving antennae to detect animal location using the antennae&#39;s proximity to the animal. Other inventors report methods to detect animal location in a rectangular cage by employing infrared transmitter and receiver arrays; these inventors include Czekajewski, et al (U.S. Pat. No. 4,337,726), Mandalaywala, et al (U.S. Pat. No. 4,574,734), Matsuda (U.S. Pat. Nos. 5,608,209 and 5,717,202) and Young (U.S. Pat. No. 5,915,332). Sakano U.S. Pat. No. 4,968,974 also proposes an infrared position detection system for an animal in a cylindrical cage. 
     An advantage of the present invention is the provision of an inexpensive apparatus that can easily adapt to the conventional laboratory animal cage, the so-called animal home cage, without any special enclosures or modifications to the existing cage. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide an apparatus which is inexpensive, can easily adapt to a conventional animal cage, and has a measurement method for determining the laboratory animal&#39;s location and movement in the cage. The apparatus is comprised of a plate placed on the bottom of the cage whereon multiple electrode pairs are configured as a two-dimensional electrode array, an electronic circuit detecting and measuring the capacitance between said electrodes, and a microprocessor determining the animal location. The electrodes are connected as rows and columns groups. An electric signal generator in the capacitance detection circuit sends an excitation signal to the electrode array. The capacitance detection circuit receives the signal from each electrode row or column group in a sequential manner. The signals received are amplified, rectified, filtered and sampled by a microprocessor. When the animal is present in the cage and above the electrode plate, the signal on the electrodes induced by the excitation signal is altered due to capacitance change caused by proximity of the animal body. 
     The microprocessor compares the signal with the pre-stored reference signal to detect the capacitor change. By determining the capacitance changes among the electrodes, the animal&#39;s x-y coordinate can be determined. 
    
    
     
       BRIEF DESCRIPTION  0 F DRAWINGS 
         FIG. 1  is a drawing of an animal enclosure according to teachings of the present invention. 
         FIG. 2  illustrates the electrode plate structure. 
         FIG. 3  demonstrates the row and column electrode connections. 
         FIG. 4  shows some samples of electrode configuration patterns. 
         FIG. 5  visualizes the relationship between the electrodes and the capacities resulting from the animal body. 
         FIG. 6  is an electronic schematic diagram illustrating the circuitry of an embodiment according to the teachings of the present invention. 
         FIG. 7  shows the two configurations of the capacitance detection input and the excitation signal output. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Referring to the Figures, the preferred embodiment of the present invention is described in detail.  FIG. 1 , a cage or enclosure  1 , usually made with transparent polymetacrylate-glass (Plexiglas) material, provides the laboratory animal  5  for observation and evaluation a bounded activity space. An electrode plate  10  is placed on the bottom of the cage supported by the fastening stands  3 . The electrode plate  10  and its electrode arrangement will be described in detail later. On the bottom side of the electrode plate are the electronic components  20  and microprocessor  30  for detecting animal location and movement. The cage is open at top and is secured by a top cover  2  made of Plexiglas or metal with ventilation openings and food/water delivery attachments, details of which are beyond the scope of this invention. 
       FIG. 2  illustrates the structure of the electrode plate  10 . The rectangular-shaped supporting plate  17 , whose dimensions match those of the animal cage floor, is made of electrical insulating material. The flat electrodes  11  and  12  are laid out on the surface of the supporting plate. The electrodes are insulated from each other and separated by a small predetermined space. Note that the figure is a simplified drawing to illustrate the electrode arrangement, and the dimensions may not be drawn to scale. The electrodes are connected as rows ( 11 ) by the wires  13  and columns ( 12 ) by the wires  14  as shown in  FIG. 3 . The electrodes connected in rows are paired with neighboring electrodes connected in columns to form the electrode matrix. Neighboring row-connected and column-connected electrodes may be further intertwined into inter-digitated patterns to increase the sensitivity of animal detection; example embodiments are shown in  FIG. 4   a , the comb-like pattern, and  FIG. 4   b , the spiral pattern. The intertwined electrodes can be made using the printed circuit board (PCB) technique. The wires  13  connecting the rows of electrodes  11  are further routed to a row multiplexer  21  while the wires  14  connecting the columns of electrodes  12  are further routed to a column multiplexer  22 . As shown in  FIG. 2 , a thin insulation layer  18  is on top of the electrode plate to prevent the animal&#39;s paws from directly contacting the electrodes and also to physically protect the electrode array from damage by animal paws. An electrical conducting sheet  19  is on the backside of the electrode plate for shielding the electrode plate  10  from interference of other objects which may be close to the bottom of said electrode plate. The electrode plate is connected to the shield signal from the shield signal driver  25 . 
     In case the animal is absent from the cage, there is capacity  7  existing in between each electrode and surrounding electrodes as shown on  FIG. 5 . When the animal is present in the cage and above the electrode plate, there are capacities  6  in between the electrodes and the animal body. The capacities  6  are in parallel with the original capacity  7  in between the electrodes and as a result, the total capacitance of the electrode under the animal body increases, relative to the capacitances of the surrounding electrodes. By detecting the capacity changes of the electrodes connected in rows, the animal location on the row ordinate can be deduced. Using the same method, by detecting the capacity changes of the electrodes connected in columns, the animal location on the column ordinate can be deduced and thus animal&#39;s x and y coordinate on the electrode plate is determined. 
     The capacity detection means is shown on  FIG. 6 . The electrodes connected in rows through the wires  13  are connected to the multiplexer  21  and the electrodes connected in columns through the wires  14  are connected to the multiplexer  22 . The multiplexer  21  and multiplexer  22  are controlled by the microprocessor  30  in such a way that only one row or one column of the electrodes is routed to the capacity detection circuit  20  at any moment. When the apparatus starts, the first row of electrodes is routed to said capacity detection circuit. Then each row of electrodes followed by each column of electrodes is routed to the capacity detection circuit one by one, separated by a predetermined short time interval. After the last column of electrodes is executed, the procedure repeats again from the first row of electrodes. The time interval in between each route is determined by the capacitance data sampling rate. 
     The excitation source of the capacity measurement is the oscillator  29  which generates high purity sine waves at 120 KHz at the preferred embodiment. The excitation wave signal is delivered to the electrode plate through multiplexer  21  and  22  after it is amplified by the amplifier  24 . The signal received from the electrodes is also routed to the amplifier  23  by multiplexer  21  and  22 . The relationship between the excitation signal and the received signal is shown in  FIG. 7   a  and  FIG. 7   b , and will be described later. The amplified received signal is rectified by a rectifier  26 . The rectified signal is then sent to a low pass filter  27  before it is sent to the analog-to-digital converter (ADC)  28 . The low pass filter  27  removes the high frequency interference and limits the signal to a low frequency band representing the animal movement by a predetermined cut-off frequency. The ADC  28  converts the received signal into digital form. The sampling rate of the ADC  28  is at lease twice the cut-off frequency of the low pass filter  27  to avoid the sampling alias. The digitized signal is sent to the microprocessor unit  30  for further analysis. The microprocessor unit contains an associated memory block  31 . The data sampled from each row and each column of electrodes when the cage is empty is stored in the memory as calibration reference. When the animal is present the animal&#39;s body sitting on the electrode plate changes the electrodes&#39; capacitance. The microprocessor unit  30  computes the differences between the data derived from the rows and the columns of the electrodes and the corresponding pre-stored reference data on the memory block  31 . A larger difference indicates a larger variation in the capacitance change in the row or the column of the electrodes. The animal&#39;s location is determined by measuring the center of mass based on the data difference. To avoid interference from other objects under the electrode plate  10 , the sine wave generated from the oscillator  29  is delivered to the shield layer  19  on the back side of the electrode plate  10  through an amplifier  25 . 
     The relationship between the excitation signal from the oscillator amplifier  24  and the currently selected electrode, which is sending back the signal to the capacity detection circuit, may be configured in different ways. The preferred embodiment is shown in  FIG. 7   a . The excitation signal from the amplifier  24  is connected to the current selected electrode  15  and the receiving amplifier  23  through a resistor  32 . The other non-active electrodes  16  (not being selected at the moment) are connected to ground. When an animal is present above the currently selected electrode  15 , the capacity between the animal body  2  and the current selected electrode  15  shunts the excitation signal to other grounded electrodes  16 . As a result, the amplitude of the received excitation signal drops at the input of the receiving amplifier  23 , and the microprocessor senses a decreased data value in comparison with the reference data in which no animal is presented. The microprocessor can determine the animal&#39;s location based on the x-y coordinate of the electrodes which exhibit the decreased received excitation signal. 
     An alternative configuration of the excitation signal and the current selected electrode is shown in  FIG. 7   b . Currently selected electrode  15  is connected to the receiving amplifier  23  providing the input signal to the capacity detection circuit. The excitation signal from the amplifier  24  is connected to other non-active electrodes  16 . When the animal is not present, only a small amount of excitation signal is coupled to the selected electrode through the capacity  7  in between the electrodes (see  FIG. 5 ). When the animal is present above the selected electrode  15 , the amplitude of the coupled excitation signal delivered to amplifier  23  increases due to the adding of capacity between the animal body and the electrodes. The microprocessor can determine the animal&#39;s location based on the x-y coordinates of the electrodes which exhibit the increment of the received excitation signal.