During three-month period ended December 31, 2018, the Company issued 250 shares of common stock for the conversion of approximately 250 shares of Series B Convertible Preferred Stock, respectively (See Note 11).

Restricted Stock

The Company did not grant or exercise restricted stock during the three-month periods ended December 31, 2019 and 2018. Stock-based compensation expense attributable to restricted stock was $757 and $203 during the three-month periods ended December 31, 2019 and 2018, respectively. As of December 31, 2019, there was approximately $827 of unrecognized compensation expense related to restricted stock outstanding. On November 23, 2019, 500 shares of restricted common stock held by the Company’s former president became fully vested upon his passing.

Warrants

No warrants were granted or exercised during the three-month period ended December 31, 2019.

Stock Options

As of December 31, 2019, there were stock options outstanding under the Company’s Second Amended and Restated 1997 Stock Option Plan and the Company’s Amended and Restated 2013 Incentive Stock Plan. Both plans were approved by the shareholders. The plans granted specified numbers of options to non-employee directors, and they authorized the Compensation Committee of the Board of Directors to grant either incentive or non-statutory stock options to employees. Vesting periods are established by the Compensation Committee at the time of grant. All stock options outstanding as of December 31, 2019 and September 30, 2019 were non-statutory stock options, had exercise prices equal to the market price on the date of grant, and had expiration dates ten years from the date of grant.

Stock-based compensation expense attributable to stock options and warrants was $(160) and $378 for the three-month periods ended December 31, 2019 and 2018, respectively. As of December 31, 2019, there was approximately $1,238 of unrecognized compensation expense related to unvested stock options outstanding, and the weighted average vesting period for those options was 3.96 years.